Rally not much to look out for other than checking all the knobs and both channels to see if it sounds and works well, though a record of what was done to it when it was last serviced would be a good thing. You could take a look at the tubes and see what kind they are. I would check and see what kind of a speaker is in it, too. A 66 should come with a CTS that has an Ampeg sticker on it or a JBL D130F. If it doesn't have one of them, it doesn't make it a bad buy or anything, but you might be able to use it for leverage to get a few bucks shaved off the price.
Ampeg Portaflex Club #1