I have a 1991 subaru legacy wagon. I just went to

Here's a video that should help https://www.youtube.com/watch?v=dKKXVOBmWEA

The speed sensor is located on the transmission output . I don't think the problem is in the dash. Go to YouTube and type in where is the speed sensor on a 1991 subaru legacy and it will bring up all related videos. Hope you find this useful.

Yes these cars are the same as far as parts go.
You will need to change some wiring on the automatic car for the manual swap to work without changing the wiring harness.

Body parts yes, but not all engine part are interchangable.

Sounds like the clutches for 3rd Gear are worn out. Need to rebuild or replace the transmission.

Sounds like you have a broken belt for the alternator and power steering.

Lubricant may not resolve it sounds like it could be your water pump froze up. Look to see if it is turning. Even if it turns it still could be bad.Usally the first sign of it going bad. Stop by your local auto parts store to see if they agree.

On the oil drain pan on the bottom of the engine.

This model does not and uses a hydrallic lash adjuster that works something like the old lifters did. Generally when these collapse or lose hydrallic pressure,a rattling occurs in the valve train. Most generally this is a sign of an oil pressure deficit and may require a new oil pump then hopefully all lash adjusters will pump back up and close the rocker to valve clearance stopping the rattle.

They recommend timing belt changes on Subarus every 125K miles.