I'd avoid the petrol ones - head gaskets are not exactly their stong points. The TD is a hoot

, but the D is acceptable if you're not into hammering round country lanes, and will do 50+ MPG.
Expect some knocking from the rear on any you test drive - this will be the subframe mounts. 1/2 day to replace, but worth the effort, and a good bargaining point! The dash bulbs will probably want replacing, but again couple of hours work will sort that.
They tend to rust behind the PAS tank, and ours is starting to go in the corners of the boot as well, but should be easily sorted with the MIG.
If you can find one with rear discs, so much the better - cars with rear drums can be less than inspiring on the brakes, even when they're in tip top condition. If it does have drums, try and find out if the rear wheel cylinders have been replaced - they start leaking, and changing them is a pain (another half day, but bank on new pipes as well!).
Headlights are pants, but not bad if fitted with Vision Plus or similar.
There's plenty about, and even good ones are cheap. Buy carefully to avoid heartache later.
They really are good cars, and very underrated (hence the price). Folk rave about the 306, but check out the prices, buy a ZX and look smug 8)