Sorry mate, had a bad day driving a long journey. I did come across very grumpy last night
Let me make up for it with some sound advice..
When it comes to being new to a different Alfa model... Keep it simple
Although every Alfa engine and gearbox combinations have their lover's, hater's, good points and bad... There are some "safer" choices out there that will give the minimum amount of ownership fuss for the beginner.
156 strongest engine and gearbox combination choices are the 1.8 and 2.0 Twin spark with the manual gearbox. The 2.5v6 24v and the 2.4 10v JTD... both manual... All offer excellent reliability and less complication than the other model variant's
However... Looking at the price range and age that you refer too:
It will be a face lift model.
It looks like the choices would be 2.0JTS, 1.9 16v JTDm, 2.4 20v JTDm and (2.5V6 24v Q-system pre-face lift model only)
Personally through experience of driving, working, fixing, and repairing them.... I would initially avoid the 2.0 JTS, and 1.9 16v JTD and anything fitted with Selespeed gearboxes not because they are bad cars.... Simply because they are overly complicated and expensive to repair for a new owner to 156's in particular the regular issues they suffer from.
JTS (fuel injection system issues)
1.9 JTD's (EGR, Map sensor and turbo hose issues)
Selespeed (cool piece of kit but the pump can be pricey and complicated to resolve any issues)
Also bear in mind that face lift models tend to have more issues with corrosion too, more so than older pre-face lift 156's... It should be the other way round really given the age of the 156 models
Both wear very well but some tend to rust underneath quicker.
Avoid looking at cars with certain mileages to... Do your homework in relation to model and engine choice service requirements... It is far more cost effective to buy a higher mileage (75,000+) 2.5v6 than a low mileage (55,000) mile as an example... This is because you know the cam belt would have been changed already and probably the clutch too by this point saving you at least £1,000 on both items that need doing on the low mileage car within the first year of you owning it...
Therefore, my choice for you would be:
a 2002 (interior face lift model only) 2.4 JTD 10v (150 bhp) with 90,000 miles cam belt , water pump, auxiliary belts and clutch done recently. Preferably from a true Alfa enthusiast as you know the bodywork and interior would be in good condition and loved.
Also avoid re-mapped and lowered, uprated suspension cars too... You simply don't know who has worked on these modifications so buy an unmolested version...