Buy an early example, 1983 high tourque.. Low kms knowing an RD350 nut helps if they have a mechaincal engineering degree even better. Time it, tune it & service it regulalruy to enjoy the full aspect and extraction of power.
Respect it because it will send you for a spill, I have been for one and took me 12month+ to recover. Modern aftermarket parts will give you control but might compromise weight. If you can carbonfire components, use an aftermarket exhaust, suspension, disc brakes this bike could be a dream with a large rear rubber of course. I would keep it original and learn how to ride it, own it and keep it. After riding many bikes that include Gixes 750’s Daytona 675 and presently an 08 NINJA 10R a RD350 still brings a sparkle to my Eye!!