Great!
Rough running and white smoke generally = glow plugs and/or pump timing retarded, assuming engine mechanical condition is sound and fuel quality is to spec... are these issues present only immediately after startup or constant? Glow plugs would be primary suspect if smoking and skipping on startup but clearing up and running good after first minute or so; timing more likely culprit if symptoms persist for a while. Persistent single-cylinder miss and smoke can indicate incorrect valve adjustment or injector fault; if this seems to be the case you can try to isolate the affected cylinder by killing injectors one at a time (carefully crack open injector delivery pipe union with engine idling -- keep hands/arms/face away from fuel spray) and if a weak cylinder is identified, proceed with further diagnosis and repairs accordingly.
Air or severe restriction in fuel system can cause runability and emissions symptoms, do you see any air bubbles in the translucent fuel line running from fuel filter head to injection pump inlet? Have a fresh fuel filter on it? Any difficulty maintaining idle speed after revving, or unwillingness to restart shortly after shutting off?
Two members here just in the past two weeks have suffered catastrophic upper engine damage due to either old timing belts running beyond their service life, or flawed belt installation causing slipped timing. If it's a lot of white smoke you're seeing, and doesn't clear up very shortly after starting, then timing is a major question and if you conclude it's not timed perfectly, you can assume the rest of the belt installation (particularly torque on dampener bolt!) isn't perfect either -- anything less than perfect puts the engine in jeopardy. Not a risk worth running, as other members will attest; if there is any question at all in this area you may want to check it out before running the engine too much more.
Oil pump flange gasket is a chore to replace with engine in car but is not a particularly common failure item. You're sure it's not from the front main or cam seal? Any oil wetness inside the timing belt cover? If it is the flange gasket, it's a good amount of work but nothing tricky, hardest part is getting the pan down enough to get the pickup tube bolts out, requires hoisting engine and/or dropping crossmember. Will take some time but doable nonetheless, and the gasket is the same as used on a 5cyl gasser Audi, i.e., readily available. There was a member here who did this within the past year or two, might try searching for posts on the subject and getting in touch to see if he can offer any tips if you need to tackle this job.