My advice, marry a rich woman, then retire to an island in the Caribbean.....
But if you insist on racing then....
Build your own so that you know what is in it, how it's put together. I've seen some designs out there that look okay, but wouldn't hold up to a high speed rollover. Besides, if you have blood, sweat and tears invested, it means more when you cross the finish line and gives you a bit more desire to finish...
Now if you have no skills, then buy a RZR XP, give Jagged X 10 grand and trust their skills...