For me, an affordable classic. DT's are great but vary on what cc you go for. 175's are very popular (if you have a full license) and demand is high for all the old classics, for all mainstream manufacturers. I would say that you are best to look for one with a V5 and as complete as possible. Some parts are hard to find and a lot have been taken off road and abused. a good, straight frame is key and matching (or close) engine and frame numbers are sought after. You can change or replace anything except the frame (otherwise it is a different bike). Only look for a "rough" one if you have the cash, time, dedication and ability to work on it. Evilbay is a good place to price up bikes and parts but decide if you want an original or if you don't mind having non stock plastics etc on it.
Write a list of what you want from it. Show winner, fun, longevity, building mechanical skills, to keep or not....... remember to factor in any tools, lubes, oils, paints etc you would need.
Just keep away from Chinese ones.