I've decided I want to get a BMX bike to convert to a Mini-Velo, which pretty much means getting a long seatpost, stem, and handlebars so it rides like a normal bike. Here are a few examples.

My reasoning for it is that I want a lightweight bike with an internally geared hub, and the cheapest one out there (Sturmey Archer 8-speed) works best with 20" wheels. Plus it seems to be cheaper to get a lightweight BMX than a lightweight road bike.

I've figured I probably want a race frame, but I'm not sure what other things to look for, like size. I'm 5'11", and weigh 150 LBS, so it sounds like I'd need 21"-21.5" for normal BMX, would it still be the same in this case?