It's exactly how you see it. Pay me now (the higher cost of the P210) or pay me later (the higher operating cost of a Skymaster, simply because it's a twin, not because it's greater than other twins).
The reasons why P337s cost less vary. There's greater demand for singles, in part because of their lower operating cost and in part because they are simpler to fly. The 337 is sometimes viewed as a "bastard" aircraft (without the allure of a "regular" twin), and on and on. My view is simple: don't worry about it and take advantage of it. You'll end up with a magnificent aircraft and the extra safety of 2 engines, without the disadvantage exhibited by regular twins if you ever lose an engine on take-off.
No specific years one should avoid, no specific equipment that is a must have, and the things that should be inspected (along with lots more info for prospective buyers) may be found at
www.skymaster.org.uk/purchase.asp
Ernie