You can upgrade every car - besides trucks, golf carts (there is a list of cars that are unmodifiable) - you just need to acquire them in the correct way. You can take an Audi and do it up purely because it is not a super car. Any car such as a Bugatti, Ferrari or the such need to be bought because if you could just steal one and sell it it will be a too easy way of making money (I'm assuming a Bugatti/Adder will be the most expensive, around $40K to sell if it was possible w/o buying it)

Go to LS Customs and it'll show the requirements for each upgrade. Also each car type has it's requirements. EX: For sports car upgrades you need to win races with that type of car (sport). Same goes for the rest.

Get to level 100 and complete idk around 100 races in each type of car, also High end cars such as Ferrari require you to buy them offline, Those type of cars cant be sold because they're are worth more than 50,000 so any car you modify to be worth 50k+ you can't sell, well thats what im assuming