I was originally looking at R53 only, but as a first track car I think an R50 would do as well since I'll be working on improving my skills. Any input on R50 as a autocross/hpde car?

 

A R50 will give you an understanding of handling characteristics of the MINI. With a few chassis upgrades, you'll have a very capable little car, but you will eventually crave more power.

 

If you want a first track car i would recommend a 90's Civic , Integra or Miata. These cars are cheap to run and you can really learn driving skills.

 

I'd prefer an R56. I've heard that the R50 transmissions aren't that robust to stand up to track use. The R56 uses the same transmission in the Cooper and S. Its stood up to our use anyway. The R56 is also top of its class in autocross, as long as the classes don't get munged again.