I'd go with a stock 46-65 2 stroke without a pipe, or a 91 4 stroke on low nitro. Anything more than that and you're going to break out. Personally, I think I'd just get the 46AX and be done with it. The four strokes take some getting used to and are more expensive up front and to repair. The pipe is just something else to break or give you trouble.

A stock 46 AX in a WM T-34 will run just under 100mph. The same engine in a P-51 with retracts should pick up a few mph. That should be plenty fast for a Bronze racer. I'd worry more about getting the plane setup for racing and less about the top speed. Get out to the field and practice until you can't stand it anymore.

Since it's your first racer you'll be lucky to go through an entire season without crashing. Don't put any more money or effort into it than the bare minimum. Don't get attached to any plane you plan on racing.

I'm not trying to put you down in any way. I've been learning the hard way myself. I raced boats for 10+ years and barely had any accidents let alone lost one. I flew for 4 years before I decided to try out racing. Over the last year I've totaled 3 WM T-34s. Luckily the airframe was the only loss on all three crashes. The first one I almost totally recovered and did a bunch of beefing up on. The second, I only added a little color to and only beefed up a couple areas. The last two that I built I added a little color and that was it. Like I said, I'm learning.