I'm not familiar with organs in the USA. But maby some little pointers I can give.
First you've to decide your style (like classic, church or rock or something I didn't know about). The sound of Hammond organs are very specific and not mine taste, I wouldn't choose that for classic or church music.
However some things to check:
- Does all the keys work (test them individual)
- If it has pedals then test them as well
- Check all registers undividual and some combined
Most old organs get a bit dull the sound is not as bright/sharp and well-toned as used to. They can fix that by cleaning the electronicparts and tune it up using potenional meters inside the organ.
In the early years they had analog organs, now it's all "digital", there is a very noticable difference in sound. Digital tends to be better, however there are some analog organs that some people think are still better then todays digital. (like Heiliger organs, they are expesive as well if you can find one!) My own organ is Johannus (www.johannus.com) but my teacher has a Heiliger and I've to admit it sounds very well (as do my organ of course
Play or let someone play some pieces on organs you may fancy. I my self find action less important on a organ because I like to play pipe organs as well and their action is nothing like a electronic organ. I prefer a bit heavier action because a pipe organ be quit heavy as well. Gives you more control as well.
And the looks of the organ are important to me as well since it takes up a lot of space in your house.