Air suspension: This is probably rear air-lift shockers, or Monroe Ride Levellers.
Easily confirmed by a 'tyre-valve' type inflation point in the load-carrying area in the back of the car. Or, you could look underneath, and view the shockers - this system has air bellows at the top of each of them. Most diesels are GLS spec, so being a comparitively rare Elite, it may have self-levelling suspension.
Chipping: A definate 'must do'!
The BMW M51D was detuned to 128 BHP for the Omega, but chipping sends it back to BMW's lively 143 BHP. It costs around 70 quid, and it's generally accepted on here that fuel economy improves too. Mine returns a proven 33 mpg running locally here.
'PM' TheBoy or Omegatoy for details in the first instance.
Engine oil/filter-changes must be done every 4500 miles absolute max; and water pumps have been known to shed their impellers at 40-50 quid a time. Other than this, the lump's a pretty strong 'un, and some have been taken to inter-galactic mileages - literally!
Best driven 'like you stole it' - it's a 'long-stroker' and must be kept 'on song' to perform. As long as you have no poor starting, (either hot or cold), excessive exhaust emissions, or overheating problems, you should be okay. Once warmed-up, the temp gauge needle should settle at just below mid-point on the gauge.
Many of the niggles with these diesels have been the result of inadequate and/or poor servicing earlier in their lives.
Good luck!