Okay so I don't know a whole lot about bikes but I've learned a little bit on this forum. I've just bought a older trek 850 21 speed mountain bike that I want to convert to a touring bike. It needs a new rear derailleur because a tooth is broken off of the one that's on it (I can still ride it) and I don't particularly like the grip shifters on it, I prefer thumb shifters, so since I have to replace the rear derailleur anyway I was thinking about changing out the shifters too. Can anyone give me ideas about touring friendly derailleur shifter combos I might like?