First up, we have a Jane Slalom Pro (3 in 1 thing). Do not buy one of these, they are very expensive and rather crap. We used the car seat maybe two or three times before it was simply too small, and our lad was 3 months premature so even when he came home he was still only 5lb - I can't imagine a term baby using it at all. The other two attachments are largely ok; but the chassis has squeaky wheels, oscillating steering and a disc brake for which I have never seen any reason at all. It's also bloody huge.
Car seat: Worth getting one with a base that belts into the car and a seat that clicks into it. Much less faff - seat is out in an instant, base can be moved to another car in 30 seconds. I wouldn't bother getting one that fits onto a pram chassis - it's not ideal for them to be stuck in car seats for long when they're small and IMO it's unwise to be paranoid about waking them; lifting from one to the other has never been a problem for us.
Pram: We used our pram for maybe six months, so I'd guess a term baby would use it for three (?) - so don't fret too much.
Pushchair: Worth getting a decent, sturdy, comfortable, padded one at first since they'll want good support and they'll fall asleep a lot, and it'll be usable on rough ground (3 wheels generally helps since for really rough stuff you can lift the back and wheelbarrow it for a few yards - gets over tricky things much more smoothly). In time (6 monthsish) you'll also want a really compact folding one - we got one for £12 on Amazon and it's been great. Ideal for shopping trips and walks on the flat, but they're not good for falling asleep in or going over bumps.
So if it were me I'd look for a 2-in-1 pram/pushchair with the pushchair bit as a priority, and get a separate car seat with base. Then get a compact pushchair when they can sit up reliably.
Isofix, no idea.