If you want to sell your donator vehicle ingame, you will have to find out ICly what people are willing to pay for it.
If it is just about bragging, simply chose whatever vehicle you think people will be jealous of.
If it is about the looks, well, matter of taste, but you can find lists of GTA vehicle via google. Then you just select one of them.
You can check the usual prices for all available vehicles in the car dealerships (Montgommery opposite DMV, Dillimore near DPD).
Also: Think about the fact that the more expensive a vehicle is (when you buy it ingame) the more the fixing/respawn will cost.
So you either drive it rarely or you have a lot of cash on the side for all the fixing.
If you sell a car "used" you will get less money for it though (if you intend to sell it).
Sorry for not giving you exact numbers or names, but you see, there are different ways of finding out for yourself now