Been looking for a clamp-on light for my AR, and I think I've pretty well settled on an older SF 660 or 660L model light.

Rifle ain't got no rails, so no rail-mounted lights.

Is there anything I should look out for when shopping for a used one?

If you are running standard handguards you could always use something like the Impact Weapons Components TWC mount and whatever flashlight you want, or there is the Elzetta ZFH1500 which mounts to a standard front sight base, several companies also offer a small rail section that will clamp to your front sight base which can be used for flashlight mounting as well, or there is the Mossie Tactics type FSB mount which is another neat solution for Surefire X or Streamlight TLR series lights. I only mention those other options in case they might let you place a light and a location better suited to your the position of your support hand.

While brighter and more efficient, most (if not all) of the 660L lights will use the KX4 bezel which is a sealed head unit that can not be upgraded -only replaced. The classic 660 series uses a shock isolated bezel which can be upgraded with something like a Malkoff M61 (and a fitting ring) which will bump you up from 60 lumens for ~1 hour to 325 lumens for ~1.5 hours for $40. It's also worth noting that the $20 P60 bulb from Surefire is estimated to have about a 30 hour lifespan while the Cree LED in the Malkoff should last tens of thousands of hours.

If you are going to buy one in person then definitely bring a set of fresh batteries to make sure it lights up and check that the tape switch works. It also seems that a lot of folks still think that everything firearm related needs to be cranked down farmer tight so check to make sure none of the machine screws in the mount or clamp are stripped.

anyone know what size ring for the 660? or recommend a good mount for them?  i have both the barrel mount and moe handguards with rail sections, but i'm missing the mount that attaches to the light itself

The size is 1 inch, I have OEM surefire mount that I never used, but any 1 inch mount should do the trick..

thanks!