You didnt mention it but have you got a charger already? And as for lipo's, Hobbyking.com sells Turnigy Nanotech's which are excellent batteries for the money. Since SMC or SPC don't sell 6S, you may want to have a look at that website.

As far as your list goes, considering what you're doing with this buggy, I would pick up at least 1 or 2 extra sets of tires, (Ofna belted street tires is something you should look at) and maybe another wing and a body.