If you're looking for a bike to experiment on why not pick up something common?The YDS is rapidly becoming a valuable antique, it's a design classic.I suppose it depends on how you are viewing it , either as a cheap bike to play with,or, as a complete and utter freaking Bargain! On UK Ebay I reckon it could get 4 times that, as it is.
Cool DS7 project. Do yourself a favor...use the later RD250/350 7 friction plate clutch (you really only need the center hub, but it's easier to find a complete assembly used). Yamaha updated the clutch in late '73-early '74 . Even a good running 250 will make a 6-disc clutch slip, especially w/ porting and or pipes.