I'd go for the Hyper 7. Parts are plentiful and the buggy drives well. For an inexpensive setup, I'd stick with the MM (disconnect the internal bec) and run it on 4S lipo. If you can rig it, the trakpower packs would be perfect. For a motor, I'd go with an 8XL or 9XL.

When I first started all of this large scale conversion stuff, I was worried about being fast. Since I've gotten settled in, I've found that it doesn't take much for an electric to be competitive on a track. In my vehicles, I've gone from the 7XL, to the 8XL, and am now running the 9XL. What I like about the 9XL is that it is very gearing friendly. I just started running the 9XLs and have found that I can go from sedate to being too fast for our track with just gearing changes.

Since you are running 15 minute mains and just want to be competitive on the track, I'd have to give some serious thought to a 9XL; however, if you do want it to be a bit faster, go with the 8XL. I haven't done any formal testing, but to be able to comfortably run a 15 minute main, I think that a 5Ah pack would do you nicely. If not, you could try the 6Ah packs, but I haven't had much luck with those.

As for gearing, I'd say that 16/46 would be a good place to start for the 9XL and 14/46 for the 8XL.