You have the best plan there is: Talking to the driver you know so he can get lessons! :) Kudos! :)
Whomever works with him may have leads on a cart, harness, and even horse. What type of cart and harness you buy is dependent upon what type of driving he wishes to do.
I always recomend starting off with an easy entry cart, they run about $450 new (I like the Frontier for a starter cart), but you can often find them used. Check Craigslist, too.
For showing, again it depends on what kind, but if it comes to a show cart, such as a Jerald, those are pricey. Even used I have a hard time finding under $1200. I still lack one, but really want one! LOL But the driving I do I don't need it (I do combined driving and also part of a driving drill team). Of course, if I had a show cart, then I could show in AMHA/AMHR driving classes as well.... ;)