Easier to use an internal hub, perhaps cheaper in the long run. A Nexus 7 can be had on ebay for about $125, if you're lucky. Parts and labor on a derailleur system would probably be more. A fixed front derailleur would just be friction, for what you suggest. The problem is the chainline going from a single chainring across several cogs on a frame with short chainstays. Maybe a narrow 5 speed?