Most people on here won't buy secondhand, with good reason, but my thinking goes like this:
The saw has a warranty period, say a year. If you are running it in the course of your daily work you'll probably notch up a few hours a day, say 15hrs a week on average (very much depends on what you're doing with it and how much you swap between saws). This equates to 60hrs a month, so for argument's sake 600hrs a year. If there are any early part failures, you'll know about them and can get it repaired under warranty.
You're talking about cutting up firewood, say 4 loads a year, actual saw running time maybe 2hrs a load? This gives a total running time of 8hrs a year, which isn't enough to even run it in!
My 066M was professionally repaired (new crankshaft) by a local dealer. They got the main bearing seal wrong, which isn't very visible but caused an air leak inside 15hrs running. Trouble is, that equated to milling two trees and then I didn't do any for a couple of years due to moving house, so when I started it up it died, needing a new barrel and piston (genuine parts ~£280, pattern parts ~£60) and I had no comeback to the original repairer (Spudulike diagnosed and fixed it for me).
Even if you play around with the above figures a bit to suit your circumstances, the odds are that you won't find a fault inside the warranty period.
Generally speaking, a saw which starts and idles well, hot and cold, is very unlikely to give you a problem, even with milling which is much much harder on them than what you're talking about. If it's been gone over by someone with a good reputation, who is happy to ensure that it is 'right' when they hand it over and will remedy it if not, you don't have much at risk.
Regarding bars, longer bars sap power. You don't notice so much on a really big saw but the smaller ones really suffer. Don't forget, with a 15" bar you can still cut through something just over 2ft (allow for the spikes).
Alec