Xenons are expensive to replace, camenlt, unsure why the cover would be missing, but thats dealers for you.
Aside of the obvious, check the equipment, the ICS panel (the display on the dash for the radio, aircon etc) has buttons for phone, satnav and CD but the car may not have these items, the CD player lives in the boot above the battery on the left, as does to satnav unit. The phone has a SIMcard slot on the front of the ICS. Many of these cars are advertised as having these features whereas they have not actually got them.
Colour is important, the bright red looks great but fades, there are plenty of silver cars to choose from but eg met black is hard to find.
The suspension seems to be a weak point, set up like the 156, but the 2.0 suffers less as the front end is lighter. 17in wheels look great but hamper ride quality.
Central locking motor failure is common, good bargaining point as £60 for the part, an hours labour and a few skinned knuckles to fix.
Rads are just as crap as any 155/145 and anything that is made in Germany, which is a lot, may go wrong (see headlights) even more so if it has Bosch on it.
That said I have not driven a more reliable or enjoyable car, it never fails to raise the spirits. The only reason I would sell my 2.5 is to get a 3.0.