When you did the compression check:
1) Was the motor hot?
2) Did you have a charger on the battery?
3) Were all of the glow plugs out?
4) Was the 550 hole the first one you did?
Any idea how many miles on the "in chassis" ring job?
Were they chrome rings, if the seller knows?
100 psi difference is too much, but faulty technique on a compression test can skew the readings. That being said, it seems likely that the extra 100psi is coming from extra carbon in the CC from the oil the rings are passing.
But maybe it is just that the rings are not fully seated, yet. It can take up to 10,000 miles to seat rings in a bore or bores that are not absolutely perfect.
I would beat the guy down savagely because of the obvious oil smoke (whether it real is oil or not. You must buy it on a worst case scenario) and the uneven compression. The stealership will tell you the car needs a motor on just the compression readings. If you can buy it cheap enough, take it home and run a couple of tanks of fuel through it and see what the mileage is and how much oil it uses. If oil consumption is reasonable and mileage is OK drive it for 6 month and redo the compression check.
FWIW, if there is any reason to suspect a failing turbo, check the function of the flapper valve at the intake manifold. If it does not close when the key is shut off and you lose a turbo you may be in for a VERY interesting day. Youtube "diesel runaway". Not so bad if it is a stick, but I had one on an automagic once.