Umax champ question


My son is interested in racing. We've talked and he would like to get a champ kart.
How far back year wise can I go and still have a competitive buggy.
I know nothing about them or ultramax. I definitely want to get away from PRC.
If you know nothing about champs like myself, and your Son never raced heads up a champ can be VERY frustrating regardless of make to deal with, and you see a lot more bent and broken parts. But if you insist I would agree anything past the Excentrik not that it couldn't be fast just tougher to get there and keep there.