Hello
I ended up buying (in addition to my bloody enormous and heavy, but a dream to push Mountain Buggy) a Quinny Zapp as a "buggy to keep in the boot" as it's tiny and with the car seat adaptors you just keep the chassis in the boot and clip the car seat on and off.
I spend a lot of time (saddo that I am) going to shopping centres and Homebase and the like and find it really invaluable being able to click the Maxi Cosi cabrio on and off. (The car seat adaptor for the Mountain Buggy was a faff and the buggy near done my back in heaving it in and out of the boot.)
Anyway, if like me you tend to drive to shops and the baby will spend a bit of time in the car, then yes, the travel system is really useful.
Lots of buggies (Bugaboo, Quinny and iCandy to name a few) work with the Maxi Cosi. Britax buggies work with their own carseats.
If I didn't need a sturdy buggy for walks, I would go for an iCandy Cherry, which is the lighter version of the Apple and has a carrycot available and adaptors for the Maxi-Cosi and is incredibly light and easy to fold (despite needing to take the seat off to fold). My friend has one and I do get buggy envy a bit...
If however, you don't spend a lot of time in the car and tend to walk to the shops and need a big shopping basket etc, then make that your top priority.
Also, measure the width and depth of your car boot as you should be able to get the "folded" dimensions of the pushchairs at the nursery store. Bear in mind you'll also want to get shopping in there!
If you go for the separate buggy, then I hear lots of people liking the Maclaren Techno XT because I think it's the only umbrella fold buggy suitable from birth. No carrycot or anything, but then I only used mine for two months anyway.
HTH. Have fun.