RAID 10 spare drive? They have to match.

Hard drive prices are still pretty high right now. However, if you want the best accident protection and limit down time or administrative woes if, as you state, a few years down the line one of your drives fail, then yes I'd suggest you just order another spare of your hard drives now. The way I look at it is the expense is minimal in the grand scheme if a failure happens and you can't find a replacement in a timely (or cost efficient) manner. It's much better to have a spare drive available and not need it then need it and not be able to find it.