Firstly, there was never a Tdi Omega. The diesel choices are:
2.5TD - BMW engine, as used in P38 Range Rovers of the era, and the 525tds. Heavily detuned in the Omega. Available in manual and auto form from 1994 to about 2000. Quite sluggish and uneconomical, particularly in auto form. Weaknesses are the autobox, and the fuel pump, and like all BMW engines, heads and HG if its have a poor service history. Note, the service interval is a ludicrously low 4500 miles... ...and it would be a brave man to regularly let it go that long between services. In standard form, too underpowered for the car IMHO.
2.2DTi (UK only, 2001-2003) - GM 4 cylinder lump, using the infamous Bosch VP44 fuel pump. This pump is easily damaged electronically and mechanically by dimwit mechanics cranking it dry rather than bleeding properly. Available in manual form only. Weaknesses are mentioned pump, vacuum issues, poor intake design causing regular "Critical System Malfunction" which is easily overcome with regular cleaning. In standard form, underpowered IMHO.
2.5DTi (non UK, 2001-2003) - BMW 24v common rail lump mated to a 5 speed auto. Never available in RHD form, so irrelevant here.
2.0DTi - rare (in UK), underpowered GM 4 pot lump, nowhere near powerful enough for a 1.7t car.