I'd say if you want to use one item long term to look at the pushchair function on it more than the pram function. IME the carrycots and use with carrycot are all pretty much the same - it's using it with the pushchair seat that you'll want to test out. Try folding it up and down and testing the weight. Have a look at how the size of the seat unit compares to other models, the height of it to the ground, any features like a bumper bar etc. Accessibility of the shopping basket.
And bear in mind that some of the cheaper ones which are sold just as pushchairs/strollers can take a carrycot and/or car seat (and there are a couple of carrycots which fit onto car seat adapters, which is a handy tip!) If you go into a shop to test them out, there will usually be a section of the from-birth ones with carrycot which are all singing all dancing, and then a separate section of "buggies" which are just the pushchair part. A lot of parents-to-be ignore this section and the salesperson in the shop will probably walk you right past it, but these can often be more practical long term and you might well be able to use one from birth with a few tweaks. I'd also recommend comparing the full-function/from birth pram/pushchair combos, when in pushchair mode, to these types of pushchair.
WRT buying new vs second hand, if you just want to be able to walk into a store (or go onto a website) and order something in the exact colour and style you want with the specifications you want, have a guarantee and somewhere you can go with any questions/problems, or you feel strongly about your baby using things that other babies have used, buying new is the way to go. I'd recommend an actual brick and mortar shop (best: Local independent, second best: Nursery specialist chain, including department stores like John Lewis) whether you use their store or their website. BEAR IN MIND that a lot of times a pram order will take 6-8 weeks, possibly longer in corona/post-brexit times so it's a good idea to get your purchase finalised by month 6/7 of pregnancy.
If you want to save money, want a popular brand/model, aren't fussed about the exact model as long as it has the right features, have your own car and time to look through advert listings, plan to swap to a lightweight/compact buggy second hand is the way to go. A brand new pram is a bit like a car - you lose 60% of the value of it rolling it out of the shop.
For car seats - always buy new unless you have one that has been passed onto you by a trusted friend/family member, and if this is the case check that it's still a current/up to date model, that you have (or can download) the manual, that it's not damaged at all and all inserts and bits like the strap covers are present - you can often order replacements from the manufacturer. If you buy a travel system second hand it's a good idea to get a new car seat. It will still be cheaper than a brand new pram. You can always take the covers off the one you've got with the set in order to use as spares :)
Insider tip, pushchair/pram companies always have a list of which car seats are compatible with their prams, these are always out of date, the actual list is likely to be much longer, unless it's one where only one or two specific same-brand car seats will fit, and then it's probably correct :)
Beware there are a lot of brands that advertise heavily to influencers (by way of freebies/sponsored reviews) but aren't actually that good longer term, they tend to be cheaper quality with a premium or mid-range price, putting themselves at a point in the market where they're cheaper than the absolute top shelf brands.