On Buying a used machine

I am looking to buy a backup used machine of a model they no longer make... so used is the only option. That said.... though few if any of us are mechanics ( certainly not I) a few things seem wise to check/test.
1) The obvious.... how does it stitch out programmed stitches.. straight, etc. etc?
2) How does it free motion with feed dogs down. Check ALL directions.... that's what this worm hole pattern test is about.

3) Remove the stitch plate and look underneath. Is it clean? Anything look odd to you?

It does on this one.... the metal housing above the hook assembly looks odd below the feed dogs. While everything else looked nice and clean from this view.... this metal looks a little rusty. I cleaned the rusty material off with a Q-tip and metal retains a pitted look. I need to check this out with the owner before I'm fully committed. There is no statement of rust damage in their for sale description. It might be fine... but certainly worth checking to be sure.
4. Check the bobbin hook area for cleanliness, dings, etc. This one appears fine.I'll let you know what I learn about No. 3. Friday Feedback.... The plot thickens.... NO water exposure according to the seller. Update... Jan 3rd a visit to a local dealer... to quote " that kind of discoloration often suggests water damage" even though the seller swears otherwise.

IT runs beautifully right now... everything else looked perfect... no real water damage evidence elsewhere....I may return this puppy anyway!