Usually you can use the car seat which comes with a travel system until the baby is around 12-18 months old easily :) They don't need to adapt as they are already suitable for this age range. You don't need to change the seat until the baby either reaches the weight limit (or height limit if the car seat has one) or their head reaches the end of the seat shell. If their legs hang off the end, that doesn't matter. It isn't uncomfortable for them and it isn't unsafe. Head, weight, or height matter - and it's whichever of these come first.
I would just be wary of a couple of things. A few travel systems come with a Group 0 infant carrier, which is an older weight classification not commonly seen, and these car seats only take up to 10kg in weight. They are also often smaller. That's often below a year for most babies, so I wouldn't recommend a travel system with a 10kg limit seat if you want to keep using it past 6-9 months. Look for a weight limit of 13kg, which is Group 0+.
Secondly, i-size car seats. This is a newer standard which offers a better minimum level of safety in car seats, and it uses height limits rather than weight limits. If you go for an i-size car seat, look for one with a height limit over 80cm. Some have 73 or 75cm height limits and again, you're looking at 6-9 months for these seats. 80cm should get you to at least a year and the bigger the height limit, the better. So always check out the weight and/or height limit of a seat before you buy a system.
Of the brands you mention, I would go for something by iCandy, Joie, Maxi Cosi, Nuna, Peg Perego (are you outside the UK? They usually trade as Mamas and Papas in the UK.) or Venicci. I would also suggest to look at Cybex and Britax, as they make good travel systems in similar styles to those you've mentioned.
I'd avoid Ocarro and Ickle Bubba as the car seats compatible with these brands aren't very good quality. They tend to be poor fitting for newborns, tricky to fit into cars, short lasting and may offer less protection in a crash than brands which specialise in car seats.
Of the others -
iCandy don't make their own car seats, but their pushchairs are compatible with Maxi Cosi adapters, and you can often find them in a bundle with Maxi Cosi car seats. BTW, Maxi-Cosi adapters also fit Cybex, Joie, Recaro, Besafe, Nuna and Venicci/Avionaut car seats onto them. You can also get separate adapters (for some/all models?) for Britax car seats. So you can use all of these brands with an iCandy, but may not find a bundle sold directly.
Nuna and Joie are the same parent company. Joie is the budget side whereas Nuna is the more luxurious side. Both car seat brands are well rated for safety. Some of the Joie seats can be quite upright and not that well supported for newborns. In terms of prams I think Joie are some of the best value you can buy, and certainly worth a look if you're on a tight budget. Note Joie Juva, the most basic car seat, is incompatible with the Maxi Cosi adapters and just fits directly onto three of their pushchairs to form a travel system. Their other seats are more universally compatible.
Peg Perego make good quality, long lasting seats but they aren't sold in the UK as Mamas and Papas have a deal with Cybex for their car seats instead. I know Mamas and Papas will therefore take a Cybex or any of the brands mentioned above as Maxi Cosi compatible. I'm not sure if Peg Perego are the same or only take their own brand seat.
Venicci are one to be aware of as they have two models of infant car seat. One is a Group 0 only suitable up to 10kg, but the other is a rebrand of the Avionaut Ultralite, which is a brilliant seat, i-size up to 86cm (so nice and roomy), lightweight, fantastic innovative seating position for newborns - I love this seat. So Venicci could be very good or very bad on the car seat front :)
Cybex pushchairs are once again compatible with the whole Maxi Cosi list, but the Cybex infant carriers themselves are good quality and test well for safety. I'd just caution they are heavy and I wasn't very keen on the head position for a newborn.
Britax pushchairs take Britax seats or you can get an adapter to put Maxi Cosi & co seats on them. The Britax infant seats are very high quality, very safe, good for newborns and long lasting.
One last point on converting the car seat - sorry this has been so long already 
- Some brands of infant seat can be purchased with a base to fit them into the car. The base then adapts to also fit the next stage of car seat on which might be what you've heard of in terms of adapting to a toddler seat? Maxi Cosi, Cybex, Avionaut/Venicci, Joie, and Besafe offer infant seat/base/toddler seat combos (called modular car seats) which will do this and all of them allow you to keep the toddler rear facing, which is a very good idea for safety. This can save you money, but consider whether you might want more choice in toddler seat later, whether you'll want to re-use the base for a potential second child and have a quick check on the price to buy an infant seat + base and then baseless toddler seat separately - sometimes you don't really save money at all.
Most important thing when considering a child seat purchase - check the fit in your car. For modular seats, that means checking the toddler seat as well even if you don't want to buy it yet. Fitting lists are a good place to start (and if you have an isize car, you don't even need the list, as you can be assured any isize seat will fit there) BUT it's always a very good idea to have a physical look at a real life seat in your real car so that you can get an idea of practical things like the front seat space required, the angle (for a newborn), the view (for an older baby), the height to get the child in and out of the seat, etc. And to see how easy it is to put in and out if you need to move it.
Hope that helps, feel free to ask if you have any more questions.