[MagicRat]

As you live in Michigan, the first thing to check for is the condition of the frame rails behind the rear wheels and under the trunk floor.

These cars had HUGE problems with the rails rusting out, causing the car to be unsafe (and the rear bumper falling off, too.) You can get replacement rails that you weld in, but you may consider the car not to be worth fixing since there are a lot of problems.

As for the door, try spraying a lot of WD 40 down between the window and the door frame on to the door lock mechanism itself. Something may just be seized or stuck, causing the door lock mechanism to remain locked.

Carburetors have float bowls. If the car is not run for weeks at a time the gas in the bowl evaporates. Then, when you try to stat, it will not run until the fuel pump fills the bowl up again. However, a car driven reasonably regularly, (even twice a month or so) will not have this problem.

Dashboard loose? Try tightening up all the mounting screws. They are usually hidden from sight, so you will have to look around carefully for them.

As for the flush....... the important thing is to drop the pan, clean it out, replace the filter and clean up the magnet inside the pan. Put the pan back on and fill with clean fluid. This replaces some, but not all the fluid......
Then, before you start the car, disconnect the trans cooler lines on the radiator. Start the engine. The trans will pump out lots of dirty fluid out the line you have disconnected. When clean fluid appears, turn off the engine and reconnect the line. You may have to refill the transmission itself part way through this operation.