You would be better off just finding a body if you're going to replace the frame and pump kit anyways. Find an inexpensive body you like, add whatever ccm parts you can afford, and a set of slps springs, and you have a really nice sniper for relatively cheap. I'd shoot something like that over an empire sniper any day of the week.
Op, if it's what I you want, I say go for it. You may save 50 bucks or so if you pick up a used body... I don't recommend it unless you already have a body in mind. You aren't any better off picking up a body over what you would be if you bought the prostock.