The water pump is cheap and easy to replace. But I usually wait until it leaks/weeps from the hole in bottom. I have never been stranded by a bad water pump.

I have had my 1980 300D since 1984. I really do love this car.

In these hot temps (your Texas 106) I do not think that the 100C is bad.

Go ahead and check the thermostat before replacing. If that car got a radiator then who would not do a thermostat ???

Good luck on your new find.

Oh, one more thing. The 1980 I own has a yellow 55 on the speedo because
at the time 55 was the national maximum speed.
I also think mine winds a little high at 75.
But remember, a non-turbo diesel has a maximum speed of 80 .
Except in extreme situations (downhill with a tailwind drafting a semi).
Of course in Texas there are no hills, so simple NASCAR style drafting will have to do.

__________________
80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250