Well, the first thing he's got to do is decide how much he's willing to spend. That will limit how much amp he can get right off the bat.
True in many cases, but for the fellow shopping the used market for a quality amp, that's not a very consistent barometer. There's a lot of amplifiers out there within a few bucks of each other with significant differences. There's also going to be (less significant) differences between amps in the new market at similar price points.
In the OPs case, he had a dealer who let him check out the Parasound at home. Very fortunate for him. I bought my Acurus off the auction site based on reputation, and was happy with the result, much more so than with the B&K I bought the same way. The prices were within $50 of each other.
The only indicator of available current I'm aware of is the ability to handle low impedance loads. Are there other specs or indicators?