Ash is probably somewhere between oak and hickory. Hard, dense, straight/coarse grained. Hand planing is not much of an issue, just recognize that the wood is hard and could be prone to tear-out if you go against the grain.
As far as joining the legs to the top, I would use a mortise and tenon. Probably 2 mortises, depending on width. You'd be hard pressed to find a stronger joint for that application.