Another vote for an IGH on a bike like that. A simple 3 or 5 speed would offer some variety while keeping things clean and simple aesthetically.
If a derailleur drive train is your preference, I'd say buy a road bike/mountain bike or other "regular" bike that is designed around that drive train.

I say these things on the assumption that the frame you have is equipped with rear-facing "track forks" rather than horizontal, road-style dropouts. Here are some pics to help distinguish:

Classic road dropout on left, track fork on the right.
If the one on the left is what you have, you have options.