Don't forget to check out the Staton-inc . com kits. They're chain driven and fully freewheeling gearbox. They're a bit heavy but they're definitely rugged and easy to figure out and work on. All their kits pull from the freewheeling hub rather than the spokes. Very good if you have to climb hills or ride on rugged terrain. No spoke breakage issues.
Check out my bike here http://www.motoredbikes.com/showthread.php?t=9041
That would have to be either a rackmount or if you are feeling brave and can find one a winged wheel would go on that too... course you'd have to find all the parts for it
It might also be worth finding out if the damping on the suspension is adjustable, I have found that spring rates for normal bicycling dont tend to be comfortable when you are doing almost twice the speed...