The rebuild means absolutely nothing if it doesn't include a new head. Check the head casting number by the #2 injector. It should be of the form 603 016 XX 01 where XX should be a number higher than 14; like 17, 18, 20 or 22. If it's 14, either they repaired the head or found one in [hopefully] better condition. You're really rolling the dice if it still sports a 14 head. If it's a later casting, find out whether they made the old style injectors work on the later casting or upgraded to later injectors. It's not a deal killer either way but a blank stare in response suggests the shop isn't qualified to do the job they say they did.

You might be in better shape if the seller is embellishing a head replacement as a rebuild.

Is there any warranty that transfers to a subsequent owner?

Sixto
87 300D