Engine shouldn't need a heater at 20 or 30 degrees. It should start the same as it does at 60 or 70 degrees. At least mine always have.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

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