People tend to be 'economical' with the truth when it comes to selling motors, especially about no-runners
.. I cannot see why you would not investiagte the problem after the so called 'bang and black smoke' whilst being towed, just does not make sense, plus as it also has a non-standard backbox on it, so if you went to the trouble of having that fitted you would go to the trouble of having someone take a look, then decide wether to fix of sell.. i susspect this is whats gone on, and he knows its a big fix.
When being towed, a lot of energy is placed through the crank pully on the belt, if he snapped the clutch up to bump it, its possible he may have snapped the belt. may have thrown a rod.. could be anything.
If your after body parts and interior also, then it may well be worth going for, but if its in hope the engine can be sorted cheap and you then have a runner with quite low miles for a fast money maker, i would be very weary.
Oddy.
