A very nice low mileage 03 or 04 SW diesel, probs 04 if you are looking for petrol but forget the GTA version at that price
2.5 v6 is pretty cheap too and if fuel economy is not on your buying list then it may be worth looking at one for cheer fun driving. personally i am a dieselhead, 2.4 JTD re-mapped is enough fun for me.
Best advice when buying an alfa though is to
1: get service history - service history - service history
with it, alfa live and more importantly die by their servicing. Any sign of a lack of servicing or a patchy history it is probs best to walk away.
2: Buy based on condition and history, not on years, mileage, colour or price. Don't be tempted to forgive some missing service history or dubious condition for a drop in price unless it is a very substantial drop. Find a car in good condition with the history then adjust your price for the miles/years/colour/spec.
3: Posssibly better to spend only about 80-85% of your budget for buying the car and keep the rest for repairs to stuff that the previous owner may not have had done though if you get a real good one this money can then be used for a trip down the pub instead or, as most of us do, on adding new toys to the alfa
4: 156's interiors do not really wear a lot, any signs of tattiness is defo bad
5: check every electrical item, there's buckets of them and very few cheap to repair/replace if they do not work. Any warning lights on will cost you the best part of £50 just to turn the light off without even getting the problem fixed.
6: Check the OIL, if it's low walk away no matter what the history - low in my book would be less than half way between the marks on the dipstick but some would be more forgiving than that.
7: check with owner if it has had belts done at required intervals (36k petrols, 72k diesels though best to do diesels at 60k max) and verify this on the receipts. If it is near or due a belt change then adjust the price accordingly by about £500-£600. Ignore the Alfa book saying the belts are changed at 72k on the petrols, it's wrong.
8: suspension bushes wear quite quickly, listen for clunks and knocks, adjust price accordingly.
Hope some of those pointers help