First off, welcome! Sounds like a cool car so far. It also sounds like you're new to the car hobby in general. Any chance you have a bud that does a lot with cars that you could bring along when you check out the Challenger? If so, tell him his role is to look at the car with a critical eye and not with the attitude of "wow its a Mopar!". Cars of these years have some common problems since they're all 40 years old now. First is rust. Wear jeans, etc. so you can lay down on the ground and check out as much of the underside of the car as you can. Trunk floors, lower rear quarter panels, passenger floor boards are all common areas of rust. If its got newer paint it could be hiding rust & bondo repairs. Second, look at the condition of the interior, trim, emblems, etc. If they look really rough it could be sign of a partial restoration. Plus, its amazing how much you can spend just bringing these kinds of items back to show quality if that's your goal. Few cars this old are perfect, but I'll guarantee you for every flaw you can see, there's at least one or two more you don't.
Taking the vinyl top off is not going to be a small task as the roofs were generally not painted and you're going to have to fill a few holes from the chrome trim you'd be removing, sand off the glue, prime & paint to match. A good shop could probably match the color pretty close as long as its not faded, etc. I'm a fan of vinyl tops on these cars so this wouldn't be an issue for me. Wheel well trim is held on by small screws inside the fender lip, but there's sure to be scratchs from the trim if you remove them, so plan on more paint touch-up.
As long as the price is reasonble & allows you to still have the $$ to fix the problems go for it. If they're asking top dollar and you'd still have to pour money into the car you'll have a harder decision. Let us know if you get it and post some pictures!
Later, Jim