I talked a friend out of an early 70s Swan 40 or 41 just a few months ago. It was an S&S design - the older style, tumblehome hull and a deckhouse, not the quasi flush deck style that came later.
It suffered some serious structural and cosmetic issues and still was listed for over $100K.
I wasn't aware of the tankage issue, that's a biggie, and found the accomodations less than impressive (esp by later standards) and some of the berths positively claustrophobic.
Based on that viewing I think they rely a lot on the name, and that they, as much as any boat, need a good hard look and serious survey. Also likely to be overpriced.