I don't blame you, it is a bit confusing! It doesn't help that there are two sets of legislation for car seat approval running side by side at the moment, meaning the age categories are a bit scrambled and some seats go on weight while others go by height.
First off it might be useful to have an overview of the three different stages of car seat you'll generally need:
Stage 1 / from birth / Group 0+
Baby seat
Carry car seats, always rear facing, which are generally compatible with a travel system. You can get compatible isofix bases for most of these seats, almost always sold separately, but most of them can be fitted into the car with a seatbelt too, which means you don't necessarily need a base at all if you don't want one. However, many people prefer to have a base for the convenience of clicking in and out. Without a base, you would have to do the seatbelt every time.
Stage 2 / from about 1 year / Group 1 (and sometimes group 2)
Toddler seat
These seats usually have an integrated harness and can be rear or forward facing. Some can do both. They come in two sizes: Up to 105cm/18kg (3-4 years) or up to 25kg (about 125cm/5-7 years). Some go onto isofix bases, some have the isofix built in so you don't need a base, and some don't use isofix at all (fit with seatbelt, but one time installation, not every time). This is the most complicated seat group/stage so I'll elaborate more later.
Stage 3 / from about 4-6 years / Group 2/3
Booster seat
High backed booster seats using the adult seatbelt to restrain the child until they are 135cm tall or 12 years old, whichever comes first. You definitely don't want to move into a high backed booster before at least age 3, and some recommend age 4 or 5 as a better minimum age due to bone development, not to mention behaviour (they need to be mature enough not to try and escape) or recommend to delay this step for as long as possible, by using a 25kg harnessed and/or rear facing seat. Booster seats don't go onto isofix bases ever. Some of them have isofix arms which just fold away if you need to use them in a car without isofix. So you can ignore this step in your decision making entirely :)
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Isofix bases - OK - so yes, you're correct that a Maxi Cosi seat will require a Maxi Cosi base, a Britax seat a Britax base etc. In many cases this is even more specific and only Britax base Model A will take Britax car seat Model A and not Model B or C etc. Maxi Cosi is a bit of an outlier here, as they have a very complicated array of seats and bases for compatibility, and even their website is not very clear as they simply try to push their latest and most expensive base. Best way to check is usually to go to a retailer stocking the seats and bases in person and ask them, as they usually know/have the most up to date list. The full table is a bit hard to find on the web - hopefully my upload works. It might be too blurry, in which case I'll try to send the link instead (but it's all in Dutch!)
Luckily pushchair adapters are far less fussy! The vast majority of brands of baby seat will go onto the same (Maxi Cosi/universal) pushchair-car seat adapters. Britax, Graco, Cosatto, generic/own brand use different ones, to go by the most popular alternatives.
For toddler seats - yes, some of them go onto isofix bases. What is quite popular in the car seat business at the moment is to have what they call a "modular" system so that the infant and toddler (up to 18kg) seat go onto the same isofix base. IMO - this is a bit of a marketing trick as they essentially want to keep you as the parent of a newborn as their customer for the purchase of the next seat as well. The idea is that because you have the base, you'll automatically look only at their compatible range, and not at other seats. It also seems to justify them charging huge amounts for the base because they claim that you are saving money as you won't need another one.
However - this is not as useful/true as it may seem for a number of reasons.
- Siblings. If you plan to have another baby within 4 years, which most people do, you will want the baby seat (and therefore the base) back for DC2. If it's in use by DC1, you have the choice of seatbelting the baby seat in, if it's one that can be belted, or shelling out for a second base or set of bases.
- Most toddler seats don't need a separate base. Only the modular ones, in fact, do. Some don't use isofix at all, some have the isofix fittings built in and do not need a separate base. Many comparable standalone toddler seats cost around the same, or even less(!) than the toddler seat from a modular set, and almost always less if you factor the cost of the base into the purchase. So it is not necessarily very money saving at all.
- Limited choice - probably the biggest problem. If you restrict yourself to only the seats that go on your base, then you've massively limited your choice, and IMO the toddler seat is probably the one you want to put the most time, energy, money and research into as there are so many options and features to choose between.
For example, a popular feature in toddler seats is the ability to turn the seat towards the door to get the child in and out, which is not possible for (most) modular seat groups. Cybex do it - I'm not sure if any others do.
Quite a few do offer the choice of rear facing up until 18kg now, which is good, as this is the safest option so good to have if possible. Worth keeping in mind.
Also, coming back to the idea of toddler seats coming in two sizes - 18kg or 25kg (for shorthand) - 25kg seats are too heavy to be accommodated under the isofix fitting system, so they must be seatbelt fitted. This is not a problem as you (or your retailer) do it once and leave it fitted, unlike the baby seat where you need to redo it all the time, but it means that ALL modular toddler seats are the smaller, up to 18kg size. This is usually sold as "up to 4 years", but it is worth being aware that this refers to an average sized child and means up to around the 4th birthday, so could be outgrown sooner. For a child on high percentiles (about 1 in 10) they may even outgrow an 18kg seat by about 2.5, which is definitely too young for a booster seat. The problem is then that you need to buy another toddler seat to last them until 25kg. People often say if they had known a seat which was supposed to last "until 4" would not last them anywhere near, or simply that for about the same money they could get a seat which lasts until 5-7 years instead, they would have gone for a 25kg seat to begin with, which is why I think it's useful to have the info right from the start. Of course, this can go the opposite way too and a child who is small for their age can be comfortable and safe in an 18kg seat until past 4, even past the 5th birthday. The problem is you don't really know what centile your child is likely to stay on until they are about 6 months old, so it's not a decision you can make before/shortly after birth, easier to wait until they are close to outgrowing the baby seat.
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Therefore - I'd approach this in one of two ways.
Either choose the baby seat based on which seat you like the best (or even just which one you can get on offer with your pushchair) and the combined price of baby seat + base seems reasonable to you, just remember to factor in the price of the base to the
baby seat only. Defer the choosing of the toddler seat until a later point, and don't factor in any of this "modular base" stuff. If later you like the compatible seat, great, but stay open minded and look at other options too.
Or work backwards, choose (or make a shortlist) the toddler seat you think you'll probably go for, work out if it's one which goes on a modular base and if so, get the compatible baby seat. Baby seats are much of a muchness, so this is a valid approach, but I appreciate adds a whole new level of decision making and research you might not want to do in the next 2 weeks - in which case, the first option is better.
One last point - one thing modular bases are quite good for is if you have multiple cars that you need to swap the seat(s) between regularly and you can leave a base in each. In that scenario, the convenience might be worth it as swapping the seats between cars is really quick and easy, whereas installing toddler seats in general can be a bit of a pain as they are designed to stay in the car.
Sorry for the information overload - hope it is helpful though, and do ask if you have further questions.
sometimes the modular seats that go on the super expensive modular base costs more than just buying a normal standalone seat! It's crazy.
I was just thinking who has written such a giant wall of text, nobody will ever read it, and it was me