The sprag clutch issue isn't exclusive to S3's. Pretty much any of the 1st, 2nd, or 3rd generation Triumph triples has the same issue; but it's only a problem if you don't take care of your battery. If the bike you buy doesn't have a 'pigtail' to plug in a battery tender, put one on. If it does have a pigtail, use it.
Dunno about any 'rear suspension problem'; but I suspect someone will chime in...
(PS - I have three early triples, I maintain my batteries, and I've never had a sprag clutch problem)
AMA# 289558, COP#0001, BIR#47
'97 Trophy 3, '98 Tiger, '03 Speedie