I'd go for it in a minute!! Anyone can 'restore' an XS650 when there are so many suppliers / supporters! With ebay and Craigslist nation-wide searches and patience, you'll have a fine, unique ride!
A buddy of mine at work has an SS350 and they're not that bad at all, IMHO.
The 250s and 125 Aermacchi / Harleys were two-strokes and entirely different animals. The 350 is a lay-down single four-stroke.
*EDIT* oops, my bad. The early 250 Sprint was indeed a four-stroke lay-down single. The later SX250 was a 2-stroke.All that said, ANY vintage bike can let you down if you have sloppy connections, or poor maintenance habits, etc.
And THAT being said, if you're planning to have her go far from home and/or far from other folks, I'm not sure that ANY vintage bike is the right candidate...