I agree with Scott. 7 or 8 speed is the bee’s knees. Anything bigger is too thin. Ask around on other message boards, and you’ll likely hear the same thing. I purchased my Cannondale randonneuse used (7 speed indexed rear mech) and the previous had never changed the chain or the cassette. No problems though–it still shifts quite well.
Maintenance is one thing that keeps me on a fixed gear. Unless something breaks, my costs are very low. I’ll replace the chain every year, usually following the winter (I use a KMC Z chain, which is only $10), and the cog will generally last two years.