Re: Thinking of buying a 156....
I don't think the spec's go in order really. Turismo is most basic though. Lusso has luxury stuff like leather, cruise.. Veloce has sporty stuff, like side skirts, sometimes a spoiler, lowered suspension, sports seats (sometimes leather)
Then there's the Ti, or Turismo Internazionale, which is "top-spec" or limited edition. Special wheels, leather recaro's, Veloce suspension and skirts, with the Lusso comfort things like cruise, auto-lights etc.
I'd go for the 175bhp 2.4. This engine is only in the facelift 156, which first appeared on a 53 plate. Old models have carried on being registered though, with the older engines in.
The 175bhp engine will map up to about 225bhp, and is 200 kilo's lighter than the 2002 181bhp (184 PS) 530D. It is more like a petrol than a BMW diesel, red-line is at 5K instead of 4.5ish, and it pulls right the way to the line, especially after a good remap.
They depreciate loads because of supply and demand. They have a loyal, hardcore following who buy new, and not that many people who want them second hand, hence low resale prices on new-ish Alfas. However, the market seems to have firmed up a lot in the last year, so I doubt prices will drop much in the next few years or so.
The 56 plate 2.5 you've seen will be one of the fire-damaged grey imports, and is the 'old' 156, as opposed to the new one.
My mate's just bought a 204bhp 330D, and my remapped 156 (136bhp original) keeps with him, basically neck and neck through every gear. No difference whatsoever!