Not sure why you're buying a 20v for your wife, unless you want to use it yourself?? It's a heavy old lump the 20v, and as everyone says the drivetrain is not up to much. The driveshafts go (ok they've been done here) and it has the GM box, which is not particularly good. Does make a nice sound though, for a diesel
You're better looking for a late spec 1.9. This either has the 140bhp or 150bhp engine (much better), has the superb 6-speed box from the GTA (one of the best 6-speed boxes ever made), the driveshafts are much more reliable, the front end is less heavy, servicing costs will be cheaper, and for normal everyday driving is just as effective as the 20v. I get 50mpg on longish runs in my 150 Sport, goes up to 56mpg on very long runs.
As everyone says, whatever you do, make sure the timing gear has been changed, and most importantly the waterpump with it. There is an emerging problem with the waterpump on the 16v/20v diesel engines (same waterpump). Look at the 'sticky thread' here for proof of this.
All the Veloce/Ti/Sport versions of the late series cars are good, this has the better suspension than Lusso or Turismo.
Finally to make sure you're getting the late series engines (which should be no problem in the facelift cars anyway), just check the engine number on the info plate under the bonnet. Here are the late series codes for the facelift cars:
JTD 20v Multijet 175 bhp: 841G000
JTD 16v Multijet 140 bhp: 192A5000
(many of the facelifts have this engine, up to about 05 plate)
JTD 16v Multijet 150bhp: 937A5000
(from about late 05 plate and 55 plate).
Hope that helps.