Harbor Freight sells and inexpensive Leak Down Tester. However, it does not come with and attachements for use on a Diesel Engine.
You would need to use one of the compression tester adapters and modify it to fit the Leak Down tester.

If you go to the Harbor Freight site and look up the tester you will find you can also acces the instruction PDF that will tell you how to use it.
You also need filtered compressed air with an air pressure of no more than 100psi; so you need some way to control the air pressure coming from the compressor.

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84 300D, 82 Volvo 244Gl Diesel

If they're using the wrong oil it could cause some problems.

But my problems were at about 10-15F with the dino.

No problems at all at those temps with synthetic.

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1993 W124 300D
-297K on the clock as I type this.

My 82 has compression of:

385
385
385
260
220

It starts fine in the summer and down to 40 degrees. Below 32 is a struggle, below 20 is impossible. It burns .5 quarts / 1000 miles.

Once its started, it's great. But you have to make sue you are near a black heater outlet.

If I were you, knowing you are in TX and there are other rust free cars to be had, Id pass.

dd

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

Unless you are buying this car for little more than $0.00 AND you are totally willing and prepared to rebuild this engine, I would RUN, not walk away from this car.

For one of these engines in the Texas heat, 5,000 miles is WAY too long to go between oil changes. The compression numbers are proving this. Also, I FULLY expect that this guy is lying to you about oil usage. I think the chances of this engine going 5,000 miles on one quart of oil are remote at the very best.

My recommendation is to continue your search for a car in good condition. They are harder to find these days, but they're out there.