Yea, the 120 is the more reliable of the 2 CDTi's. Don't get a 2.0 or 2.2 DTi though, there the old gen oil burners. My 53 plate 2.2 is the last of the Z22SE's and as said the went to direct injection after that. The Z22YH suffers from tensioner failure, HP pump failures and occasional inlet manifold failure also but the big one is the HP pump on the rail. If it has a Siemens pumps then it WILL fail, if its a continental one then there fine.
The one to get would be the Z20NET, the 2 litre petrol turbo, they only develop 175 stock but 200 ish can be gained from just map, and the MPG is about on a par for the Z22SE in the Vec C. The NET is pretty much uber reliable.
The 3.2 V6 GSi/Ellite could also be one to look at, and belive it or not the 2.8 V6 turbo (de tunned VXR) in the facelift Signums can be had a a very good price now. The Guys running these 2.8's are getting better MPG than my 2.2
If you get knocking on a test drive, the main problem is the drop links, easy and cheap to fix, there are other little minor things but the drop links always seam to be the first to go.
Expect to pay a premium for a CDTi over the petrols as well, so it might be worth factoring that into your total yearly spend, Car, fuel resale and so on.
HTH
Steve