Maybe that used to be true as Alfa obviously sold way more 2.5 cars than GTA but at least in the UK it isn't anymore. I've paid casual attention to 156 for sale the last few months and websites showing how many MoT'ed (road worthy test certificates issued to cars) cars of any given type are still driving around. Basically right now there aren't any 2.5 156 for sale on eBay or Autotrader whereas there are a handful of GTA on both sites.Could just get a 156 with the 2.5. Authentic engine displacement, same as when the engine originally came out. Plenty around, cheap. Less chance of head gasket failure, blowing diffs and the like. All while retaining most of the sound and revving characteristics of the 3.2 versions. Keeps the good looks, and functionality of 4 doors over the GT. Slightly shorter ratio gearbox over a 156 GTA I believe too?
Even the newest 156 is going to be about 15 years old and Alfa sold the most likely a good few years before they stopped production. My own one reached it's 18th birthday this year. When I bought my first 156 there were plenty around for £1k or even less but nowadays most of the dogs have all died off from lack of care and maintenance.
GTA are more susceptible to head gaskets? Blown diffs?
There is a massive gulf in power between the 2.5 and the 3.2, 50 to 60 bhp and a not inconsiderate amount of torque either. Not knocking the 2.5 but it's a significant step down if the OP wants a GTA.