The Aton base fix is an isofix base yes, but it can also fit with seatbelt. Isofix refers to the method you use to attach the base (or later the full car seat) into the car. You can do this with a seatbelt by following the blue guides on the seat (make sure to read the instructions, you can download them if they're missing) or, if you've got an isofix-compatible car, isofix is dead easy because you just pull out two metal prongs on the back of it, push them into holes in the seat and they clamp on tightly to a metal bar attached to the car's chassis so it's absolutely solid, really easy to do, will turn green to show you you've done it correctly. It was designed to be idiot proof and it is! There is most likely a fitting video on Youtube for the base. Then the seat just clicks on and off.
Make sure you find out if it's the base fix or base 2-fix. The base fix takes the Cybex Aton car seat and the base 2-fix takes the Aton 5. No other current baby seat will go onto it, they're specific only.
The Cybex car seats go on Maxi Cosi adapters, so they fit most popular prams/travel systems. It's not my favourite car seat for a newborn though. It's really high safety rating but I was disappointed with the head position and DS2 outgrew it fairly early, he is tall though. But the bases are £££ so it kind of makes sense to stick with what you've got.
It's actually quite a good base for a 3 door car because the release button is at the front, although the carry handle is supposed to be down which might be a bit awkward. I'd check with Cybex whether it's OK to just leave it up for ease of getting the seat in and out.
It's safe to put a child seat in the front IF you can disable the airbag, it's the airbag which makes it unsafe.