I think you worry too much about being at any disadvantage. In Formula 1 not everyone races on a fresh engine, but some do. Racing in general is made up of cars on different parts. Dan Wheldon won his last Indy 500 with a car that was 5 years old. Richer teams can afford to rebuild their engines more often, smaller teams have to run longer on the same parts.
In real racing hardly anybody has equal equipment at any time.
The point of GT Life is to give the feeling of car ownership, which includes maintenance, and which can be expanded upon. If you remove oil change, car wash, engine rebuild, and other things like that suddenly theres no pointing in owning cars. Gran Turismo should be like PC sims and have every car available from the start, all of them identical, all of them with no mileage or the ability to gain mileage. No oil change, just lifeless copies.
This means no garage. Enter a race and choose from a list of cars all with identical specs and in identical condition, like Arcade mode.
Sounds kind of boring to me.
Problem is none of these affect the performance of the car so aren't really needed.