I fancied retrofitting aircon too, using genuine Alfa parts from scrappies. I think the biggest problem would be fitting the internal aircon / heater matrix - as far as I know, it's all one unit (i.e, you can't just add a cooler matrix inside) and the whole dash needs to come out to fit it.
However, I reckon the parts list is no more than:
New belt (does it run off the alternator belt or it's own?)
Switches and wiring
Internal temp sensor (goes down by steering column)
External temp sensor (door mirror)
Air con rad (condensor?)
...and lots of patience
I'm also wondering if basic aircon (i.e., not auto climate control) needs the temperature sensors?
Last time I brought up the subject, a few on here said it's not worth it since the Alfa air con is next to useless... but I still fancy having a go.
There's always one - and it's usually me!