It will require more maintanence than a modern bike but if you're building it from the ground up there's no reason you can't make it better than new from a reliability standpoint.
If you don't go wild with the porting and cam and don't ride it at 10,000rpm all day long there's no reason it wouldn't make a nice toy. Most of the problems I hear about are from people trying to get by with old systems... like 40 year old wiring, etc...
You should get signed up to the bevelheads mailing list if you're going to be playing with this stuff, there's a lot of info available there from people that have been riding these things since they were new. go to
http://www.ducatimeccanica.com
for more info...