Hi there Cyrage, welcome to TDUc and we hope you enjoy yourself here as well as do some cruises and such with the very friendly but slightly strange and weird PC squad we have here. :D
:welcome:
As for your questions, hmm let's see:
1) Check your specs against the minimum specs posted in the TDU PC section, also posting your PC specs so others who may have or who have had experiences with your kind of setup will be able to advise you better.
2) Stages on the car is the tuning which usually results in a performance increase, it's like some tuners in real life, you can ask for a Stage 1, 2 or 3 tune for your car and the higher the number the more it costs but the more power you'll receive. So in other words Stages is the tuning of the car, the higher the stage means costs more but a lot more power!
3) Nope, mileage has no effect on the cars handling or performance, it's just a bragging right essentially.