If you can't drive it couple hundred miles right away or after replacing some consumables, i wouldn't pay more than $1000 for any of these old trucks.
Who knows the Can of worms they might have. This one sounds like it has its fair share of worms. How much is he asking?
I'm a total tow noob. when i brought my golf cart home it had a super steep incline on the interstate. Truck was WOT around 2200 rpm and 35 mph haha.
Still passed one semi hauling something extremely large and heavy, they were doing around 15mph so, relatively, i was flying! Haha.
Pick an issue and start diving in. Then come back with specific questions.
That said, id focus first on getting the engine to turn over. So you'll need a starter at minimum.
I don't see any reason why you can't drive it as long as you want. My c20 has an sbc with a turbo 400 heavy duty trans w/ 4.10 gears and ive driven it on a 8 hour road trip. I was maxing out around 60-65 mph but yeah, driving just fine.
Yeah scam shop and bad truck owner.
It's just a duramax is all, in the post i said it was much newer. Just a data point on a more modern duramax for the OP.