Low Cost Racer
The plane is normally the least expensive part of a pylon racer unless you are into the composite racers. Any Quickie 500 pylon racer will do over 100 mph. As said above, it is the engine, prop, and exhaust system that makes the difference. A Scat Cat or Dodger kit can be purchased for $35 - 40 USD. With a stock BB 46 and a 9 x 7 APC, stock muffler will run over 100 MPH (approx 120 MPH). Many areas are running a modified AMA 424 class with K&Bs, TT 46 PRO, and OS 46FX.
You can purchase the Lanier RC Dominator for about $50 USD, or if you prefer an ARF, you can go with the Viper or Predator. I personally don't care for the ARFs, though that is personal preference and you building skills. You can run a Mac header and tuned pipe and get 140 - 150 MPH out of a stock BB 46. The whole key is to build light and strong (as with all planes). You can put a JETT Quickie 500 motor on a Quickie 500 racer with a pylon racing prop (8.75 x 7.0) and get 160 - 180 mph. If you are just looking for speed, look for a Hyper. I have one with a Sport Jett 46 with a Jett mini-tuned pipe that hits about 200 mph (what a thrill!). A friend of mine is running a Nelson Quickie 500 (AMA 428 Class) with a hand carved prop that has been clocked at 220+ mph. A Hyper kit is only about $40.00 (foam delta wing).
Hope that this helps you out in some way.