Hi Henry, welcome,
I have had to adjust the front ride height on Sideways cars, and I have heard reports you need to do the same on NSR 917's (no big deal since there is provision for this on the chassis) This is because the rails on Carrera are about 0.40mm above the plastic height. This might sound like a pita but in practice its not a problem. Even when the cars drift across the slot I never notice any issues.
There are pros and cons of each of the mentioned track systems, Scalextric is more convenient for smaller spaces due to the general geometry of the track and probably a little easier to find depending where you live so might be easier and cheaper to extend your layout later using second hand pieces (new prices may not be much different)
You will need to chip the analog NSR cars which for me is part of the fun. The rail protocol is different between Scalextric SSD and Carrera Digital, so your option if you want a chip to do both is the Slot.it type C which can switch between the native protocols using an app to reload the appropriate firmware, or when it comes to market, the upcoming Scorpius decoder. There may be others in the distant future, only time will tell. If you are happy with being limited to just one system or the other then its going to be quite a bit cheaper to stick with the OEM decoders from whichever system you use. Just be careful with the factory Carrera decoders since some of the V118 27632 (and some other model numbers as well) have "aggressive" throttle response and may make the cars more difficult to drive. If you can find older production runs of the same decoder you will be fine. Some people are happy with the more aggressive throttle response so you could be happy either way depending on your preferences.
Best of luck.