40K may be a bit of a stretch unless you have the funds to kill it with advertising and if starting this year I just don't see it happening this late in the game. It will take time to get a customer base.
If your back is prone to giving you problems bouncing around on a ZTR is not going to be good on it. The extra weight should come off as you will be most likely more active than you are now. You know the situation with your back better than us and if you think it might take you out of commission then how could you service your account while you recoupe as a solo?