The only part of the ADAC score that was bad for the Maxi Cosi Pebble 360 was the "pollutants" bit and it was some tiny bit of chemical in the airbag warning label. They have since changed the way they make it anyway. So I would have no concerns about that seat in terms of safety.
TBH you can ignore the pollutants bit on the ADAC test anyway because they use different covers for the seats in Europe where those tests are done vs the UK because of flammability laws for furniture - infant car seats are classed as furniture under this law because they are part of a pram. That's why Which? uses all the crash test, usability parts of the ADAC test but they don't include the part for pollutants.
With a three door car you probably actually won't want a spin seat for the infant stage. In fact because they are so reclined at this stage, you will likely find that you can't spin the seat to face the middle of the car, because the back of the seat will bump against the wall (where the door would be) and it won't turn.
Having a base is very helpful in a three door as it is awkward to lean over/around and do the seatbelt. You can just get the baby in and out of the seat in the house, though, or pick the seat up, place it onto the front seat to get baby in and out if you don't want to use the car seat on the pram.
I agree that you definitely want to check where the position of any buttons etc are on the base, preferably in your specific car.
If you're thinking about the toddler stage and future proofing, then the Cybex will be no good anyway, because you can't operate the spin function on this seat easily in a three door. You have to push up under the seat below the child's feet and this is totally impossible to reach. You would be OK with the Britax Babysafe 5Z2 though as the toddler seat for this one is Britax Dualfix 5Z and the button to spin that one is near the front of the seat. Nuna Base Next is also OK but I would say those two are probably the only current modular spinning sets with a reasonably placed button for 3-door cars.
Again, you would want to double check that the whole system (including toddler seat!) fits in your car and you can actually spin it in the preferred direction. You often cannot return car seats so please do check this before you buy!
Another option to think about is to just buy a cheaper base and infant car seat (various models available from Cybex, Britax, Maxi Cosi and others) without spin function, and then get a spin seat just for the toddler stage with an accessible button, such as Maxi Cosi Mica, Britax Dualfix Plus, Silver Cross Motion all size etc. This is often a MUCH cheaper option overall compared to buying the modular sets - the set with rotating base, infant and toddler seats tends to cost in the region of about £750, whereas a non-spinning infant car seat + base set costs about £180-250 depending on model, and the spin seats that you just buy on their own (no separate base) cost about £250-300.
Agree that infant seat belt fitted on front passenger seat also another good option (and cheaper, as no base) if you can disable the airbag. You can get belt fitted rear facing toddler seats suitable for use in the front seat for later.
You won't get a base for the Cloud T second hand, it's too new. There are risks to buying second hand as said. It's not as big of a risk with bases but you would lose out on the chance to try in the car so a bit risky in that sense. Plus when buying a modular system it's a pain if you get stuck with an older model and then can't buy the upgrade anywhere because it's no longer sold.