You should get the car seat new because some people don't realise that you can't sell them on after an accident, or they might know but not be honest about it :( Sometimes people also sell on really old car seats which don't meet current regulations or have been stored badly. It's okay to get one from a friend or family member that you trust and can verify the history. Once a car seat has been in a crash, or dropped, it's unsafe as they are only designed to withstand one big impact, after that if they were damaged again they could collapse completely or fail to protect the baby.
Also, you need to be careful of very cheap new car seats, because some of the brands produce them to the very minimum safety requirements. In particular there are some budget brands which are all made by the same factory in France (Nania, BabyStart, Beone, I don't know what other brands, because they change all the time) - so it's worth being suspicious and checking out safety ratings. Your local library should have a subscription to Which? which is a useful comparison tool or you can look at the German ADAC tests online - you'll need a browser which translates, unless you're happy to just look at the overall score. IMO, this is less of a worry with an infant seat as they tend to be hard to get wrong, but cheaper ones might be small, meaning you can't use it for as long (which is important because rear facing is safest even for older babies) or the side impact protection may be low.
I can see a couple of options for you:
Suggestion 1: New car seat, second hand compatible buggy forming travel system
The Maxi Cosi Cabriofix is currently on offer in Mothercare for £99. They price match their website in store. This is a highly compatible car seat with loads of different pushchair brands, meaning that you could easily find a second hand buggy for £100-150 which would work with it if you keep an eye on local facebook selling sites, NCT sales, local second hand shops, and ebay collection for cash only. Just ask the seller if it works with a maxi cosi seat. Sometimes you need to buy adaptors which are about £20. This fits in the car with a seatbelt. The bases for the car are about £100 extra, IMO, it's not worth it. But you might be able to get a base second hand if you really want one.
Suggestion 2: New complete travel system
There are plenty of complete travel systems for sale new for under £250. Look at the My Child Floe, this seems surprisingly good for the price, though small wheels are not very good for going off road. Actually, if you really were set on a pushchair set with a car seat base included and could stretch budget a bit, the Nuna TAVO is on offer in Mothercare at the moment for £300 with car seat base included, which is very cheap. Do keep an eye on offers - Amazon Black Friday is coming up shortly.
Suggestion 3: New separate car seat, new or second hand buggy. But not travel system.
Buy a car seat independent of the pushchair which you'll intend to keep in the car or only use for car-to-house scenarios. (This is what I did with DS as I could not drive myself, and didn't want to use the car seat on the pushchair.) You could still get a baby carrier one if you want - the Joie Juva is only £40 but is a good buy (Joie is a decent budget brand). Or as other posters have suggested, look into getting a car seat which will last longer and stays in the car permanently. Do bear in mind that if you plan to have another child in the next 4 years, you don't actually save any money as you'll end up needing to buy a second seat for the second baby! If you have a separate baby and then toddler seat, you can re-use them. But if you only plan to have one child, the combo seat can be a good buy.
Unfortunately the combo seats are where the cheap crap seats come out in force. Be suspicious of anything with overly shiny fabric, very shallow "wings", or straps which tighten individually, like a rucksack, rather than from a single pull mechanism out the bottom of the seat. Also, if you get a rear-and-forward facing seat, make sure that it rear faces until at least 13kg. Rear facing to 18kg is even better as it gives you the choice. Some cheap ones only rear face to 9kg or 10kg which means you have to turn them forward facing much too early.
In this category I'd recommend: Joie Tilt, Joie Steadi, Britax First Class. These all last until about 3.5 years. Joie Stages, if you can stretch to it, will last you until about 7 years old.
I also recommend researching different kinds of slings. If you walk a lot this will be much more use to you than a fancy travel system.
Good luck, happy shopping!