I've never bought a transmission before...

Building a 727 to be stout behind 500hp is so easy it's pathetic. Anybody with any familiarity with one can do it. The problem is the kickdown. It's very picky and getting it right can be a great frustration; somuchso that many folks opt for the manual valve body; not a good street choice. The converter is "kinda" important, but notsomuch with your hp level. Just don't go with a very high stall unless you tub it big. Me, I would just make sure that the direct clutch pack had at least four frictions and I'd go cheapo on the shift kit with TransGo's El Cheapo minimum hd/rv type kit with the new manual valve and drill out a few holes in the valve body separator plate at least as big as recommended for the 1-2 and 2-3 shift. Then spend the rest of your money on getting some traction.