Still Time to Save This '17 Sierra With Torque Converter Issue?

.. interested in purchasing this pickup. It has a 4.3L V6 EcoTec3 engine with a 6-spd transmission. The salesman says the truck needs a new torque converter. We test-drove it about 5 miles with about 8 stop-and-goes. On our next to the last stop @ about 5 mph I felt/heard a bit of a thump underneath. The salesman said he'd noticed the same 2 other times during our trip while the vehicle was accelerating, just not as pronounced. There was no shudder nor sudden slow-down when I noticed the thump. .. nothing remotely violent. Otherwise, the truck drove just fine.
I understand a bad tc is a major issue & needs to be fixed asap as it can destroy not only the transmission but the engine as well. But I'm wondering if based on the symptoms I've described if it's likely that damage beyond the tc has occurred?
I'm also wondering if they really know that the problem is a bad tc & not something else such as the tranny or engine.
This truck cosmetically is a knock-out for a work truck, @ least in my book. .. exceptionally clean both inside & out. The dealership put on new tires, brake rotors & a buncha other stuff b4 diagnosing it with a tc problem. They'll sell it to me @ the asking price, which is lower than similar vehicles I've seen, or take $1500 off as-is.
What's the verdict?
Thanks everyone.