Find a Honda GB500....ready made late model cafe bike.
Offhand I can't think of any small displacement newish bike that would offer both decent performance and the necessary design to allow conversion to a cafe style bike. Most of the sporty small bikes have a frame and bodywork that just isn't conducive to cafe modification, or you have a frame that might work..like one of the "standard" small bikes that just offer such poor performance that it would be no fun.
And like Joe says...once the initial restoration is done on a CB350 it should be damn near bulletproof for the most part. If you don't like adjusting points put in an electronic ignition and that's about the end of maintainance except for oil changes. I mean ANY bike will require chain upkeep, filters, and the odd electrical component now and then...but a CB350...well that's about as trouble free as they come....assuming it wasn't a piece of junk to begin with.
My choice...would be something like a CB350/4. Engine built with loving care, carbs done up right, proper suspension, updated ignition and the thing should run for thousands of miles a year with minimal maintainance. Plus...they sound great and there are still hop up parts for them. Hell...a late model Ducati will cause you more problems than an old Honda.