Question, if player A steals a modifiable car, can a high level player B take it to the shop max out its parts and performance and then give it back to player A to put the tracker and insurance on it?
If yes then I would be willing to pay someone in FG to do this for me. Looking to do the Imponte Phoenix. What would the price for something like this be.

Nope, once someone costumize a car, of the player B takes it and try to go in the los santos costum, it will say "this vehicule cant be modified".