After those 2, what are some things I should be looking at/ checking/ testing once I have the machine in front of me?
Great question!
When I go to look at one I look at overall condition, which you mentioned, but many people miss things like looking inside the air box to see if it looks sealed, or if its full of dirt/mud, then at least you know it was for sure mudded, and probably not maintained the best. Even better, pop the air cleaner off and see if the intake tube is clean...poorly maintained/sealed air filters lead to a dirty intake tube, and a "dusted" top end $$. Yamaha had some real issues with this with their "drop in" air filter design used on many of their ATV's...is SOOO easy to not get it installed right and then it leaks at the front seal...I also grease the front foam seal for an extra layer of dust sucking protection.
Also, pull the dipstick to look at the oil, is it full, and clean, also smell it, it should not smell burnt, or gassy.
Now here is the big one, and almost nobody else does this, but it is eye opening in the things you will find.
Get the owners permission to stand the quad up...make sure the fuel is turned off (if it has a petcock) and put some cardboard or something down so that it doesn't scratch the rack or rear grab bar up, then get help to stand it up, some quads are pigs and might even need 2 other people to get it stood up. Once standing you can see how much mud is packed up in stuff, mud that is almost impossible to see any other way. It allows you to see if the frame is bent (more common than most people realize) and how scratched/dented up the frame and suspension components are on the bottom...cracked motor mounts too. It allows you to easily check steering and suspension joints too.
Also can check front drivetrain/Ujoint slop while its up too.
The last thing is while its up its easier to see the serial number stamped into the frame...make sure it matches the title (if it has one...older quads didn't I can't remember when that started...late 80s or early 90's IIRC)
And going back to drum brakes...if you don't regularly go "submarining" with the machine, or constantly mudding, I don't find drum brakes to be that problematic...disc are a decent upgrade though.