First, congrats on a great price!

These transmissions tend to slip on the 2-3 shift. Sometimes working with the vacuum modulator will help. (Adjust or replace).

Shocks are probably OK if they are not leaking.

I don't think you'll get that door panel off with the door closed. There is a vacuum motor for the locks. I think it's under the back seat. Maybe if you deal with that, the door will unlock as intended.

Look closely at the bushings in all the rear suspension arms and links. They wear and the car wants to go all over the road.

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2012 E350
2006 Callaway SC560