If you take one piece of advise, get the "Deluxe" shift kit, because it has all of the components of the HD shift kit but comes with a lot of useful bicycle tools, that if purchased individually they would be more expensive.
I would also purchase the optional left hand side chain tensioner and the right hand side chain tensioner, because these two items (although not required) make the shift kit bullet proof in practice; giving total reliability as well as increasing it's service life.
You will need to make these two small modifications to the shift kit for reasons that are explained:
http://motorizedbikeforum.the-talk.net/t353-additional-stability-for-the-sick-bike-parts-shift-kit
and this simple modification to the right hand side chain tensioner is beneficial:
Spare parts that i would order from the word go are 1 x replacement snap ring + replacement locking ring + replacement bearing for the heavy duty freewheel.
In my application i have found that the HD freewheel bearing fails after 6,000 kilometers (3,700 miles) but if you replace the replaceable inner bearing race after 2,500 miles you will have total reliability of this component.
The heavy duty White Industries freewheel bearing is the secret to the long term reliability of the shift kit - don't leave home without it.
A non essential optional improvement to the right hand side chain tensioner is the use of Terracycle ball bearing flanged idler wheels, and they are sexy looking items.