If you are going to do more dirt riding then my mini buggy does awesome. We did have the 650cc motor, when we bought mine that was the biggest motor they had, pd 7,000.00. Now they have bigger motor packages. We had to upgrade from the 650cc, instead of spending the 3,000.00 for a turbo, we picked up a geo metro for 50.00 and that motor works great. It would probably do better in the sand (Bob can dune it), but I am still learning how to drive it in the sand. But I love it and can't complain about anything!!
This site has a large variety of what you might be looking for including the better Joyners:
http://www.buggyworld.net/vehicles.html
Here's another site also:
http://extremecartsunlimited.com/photos/in...%20Joyner%20USA