Personally I would go at least 28 on the tires. I would also consider bar end friction shifters. You probably need different bars for that though. Velo orange has a nice selection of bars. As for the rear derailleur, it depends on how much adjustment you have. It's doable with many derailleurs

Friction shifters and a shimano xtr rear derailleur.
and not with some. To your advantage the friction shifters will work with any derailleur. Here is a pic of my bars with friction bar ends.