Starter motor should never be cranked for more than 30 seconds to avoid burning it out. But 30 sec cranking intervals is time enough for the poor-man's compression test....... without glowing the plugs, where healthy diesel can start from cold on warm day with its compression alone. If it entirely flunks the poorman's test, it doesnt mean the engine is worn out.

Diesel compression guage measures up to 500 lbs pressure. They are expensive, maybe you can rent one, or just spring for professionally done compression test.... consider readings at mid to high 300's to be excellent..... high 200's can be considered good, as burning a little oil (1000 miles per qt.)..... anything at/below 200 should be considered crummy, as in worn out engine with cold start problems and high oil consumption.

Compression test is worthless if valves are out of adjustment..... Otherwise, the guage is attached at injector holes and requires two man operation with one guy cranking the engine, while another reads the guage - noticing how fast it achieves compression, and if discrepancy at/above 10% exists between cylinders. If you pull the injectors, entirely remove injector lines first - so they dont get bent out of shape. Hazard is allowing dirt particles to enter the fuel injection system, so cap all openings with masking tape.

Other pre-purchase diesel diagnostics include studying smoke (black, blue --> carbon build up, oil, unburnt diesel fuel.... Some blue smoke that clears at start up is normal, but it shouldnt be smokin like a burning tire all the time.... also listen for metalic noises (injector nailing) at cold start. Nailing indicates dirty or worn injectors. Oil is always black as pitch, and normal healthy diesel rattle is easy to identify.

Another thing is to remove the oil cap while engine is running, checking for 'blow-by' of engine oil from crankcase. Maybe ballance the cap on its opening and see if it flies off into the air, or just rattles around barely screwed into the opening at the valve cover. If the Heckflosse diesel has modern day air filter, instead of retro upgrade 'oil bath' style, then dirty air filter covered with oil will tell the same story of crancase blow-by as the oil cap test.