Yes, I'd want a spoke rear. They handle abuse much better. 17" or 18" would both work. Depends on what kind of tires you want. For serious offroad work there's a much better selection of 18" tires. Problem is that none of the 4 lug monolever bikes had spoked wheels and therefore there is no such thing as a 4 lug monolever spoked hub, so you'd have to make one by milling down a R100GS hub to fit. You definitely want a 21" front rim, probably 1.85" wide. Forget about a disc rear on a monolever, unless you want to do a LOT of work.