420 SE
I like you are able but older and less tolerant. If you do searches most of the repairs you need have been well covered on this great site. From there you will need to decide if the repair is within your tolerance.

As far as priority you need to fix the PS leak first. It is only one of the reasons the other rubber parts failed but may have accelerated the failure.
Clean off all of the PS fluid residue before replacing the rubber parts.
Second might seem strange but I would fix the AC drain next. Unless you know where the water is going and will it will not be causing rust.
Last I would fix all the rest at the same time. Personally I would find a good indie to do this. (now outside of my tolerance except for steering damper and maybe the motor mounts) Steering damper is only 2 bolts.

I would recomend that if you replace both sides of the car at the same time.
It will not add much to the overall cost and you will only need 1 alignment afterwards.

The temp gauge is not an omen and has also been well covered on this site.
Could be as easy as the ground coming loose.

You do not state the year but a 126 is at least 15 years old. Be ready to replace a lot of rubber parts. Including the diaphrams for the AC vents and door locks.

I do not know how many miles 190k is so I would also try to find out if the timing chain has been replaced. Replacement is reccomended at 100,000 miles. Replace the tensioner and guides at the same time. This is not cheap unless it is within your tolerance but a lot cheaper than the cost after failure.

Hope this helps!
Gary