One thing you might consider doing is to buy the IWD Ultimate Collection. It has IWD, HoW, and IWD 2 in it. Yes, you've already bought the basic game, but HoW on its own is quite difficult to find. I had the basic game too, then bought the UC because I couldn't find HoW anywhere else (granted, in only about a week of searching). The UC is pretty cheap, I got mine for about $30 (including S&H) off of Ebay.
And your thief CAN get outnumbered 8-1 if he gets separated or detected while scouting.