It's a bit confusing that it is advertised as a non-runner, but then there is a video of the engine running. Is there also a transmission issue that prevents it from running?
As for the exhaust smoke, I think it could be oil burning which could be anything from turbo oil seals to piston/bore wear and damage. Unlikely in my view to be valve stem oil seals because that fault normally only gives puff of smoke on start up, as oil that has accumulated is burned off.
The amount of smoke from the exhaust, if it has been going on for a while, may have damaged the catalyst.
Personally, even at $7k, I think it presents too much of a risk