If the gun is a no name brand, then I would give it a miss. Go to some auto body suppliers and look at the brand names of the really expensive guns, do a completed items search and that should give you an idea of what people are paying for new, near new, and used guns. If you are just gonna shoot primer, it will probably do an ok job. You will also want a gravity feed rather than one with the paint pot underneath.( i found out the hard way after sparying a couple of cars one weekend, it was off balance and made your arm tire easy.
I am also going to need a new gun, but I really haven't researched anything or picked a price range I want to pay. But to get a good one you will probably pay 150-200 for a good used, or new gun. Also if you have good pawn shops that stuff shows up alot.
Deak