I think, if possible, you need to ask your surgeon or contact the specific questions about mtb'ing, ebike weight and other factors for your personal situation. I had a spinal op last year and at my last followup I asked my neurosurgeon about mtb ing. He said that there are risks, as with...
3-4k can get a really good FS ebike:
Commencal used as an example as I've just ordered one after looking at all of the alternatives or there is the vitus esommet vr etc.