I've got a Corsa 1.4 with aircon and let me tell you, it saps power big time! Although when you press down on the pedal, IE. full throttle, it turns it off momentarily, works quite well, if the car is cool inside it switches it off while I'm pulling away also up until about 40km/h. I think the cars with the smaller engines probably work in a similar fashion. In terms of cooling power, my moms Mercedes 220 CDI (or whatever Merc calls the turbo diesel) has climate control and it cools a bit quicker but the Corsa aircon isn't bad at all, full fan speed and it'll cool down the car very effectively & quickly.
Aftermarket is a mission, trust me, I wanted to do it on my older Corsa but it's just too much work, you need to either get the complete unit from a scrapyard or else have everything made custom, as in custom mounting, custom crank pulley, custom radiator mounting, custom in dash mounting for the evaporator, it's not a easy job.
Also, car radiators work via the crank, you'd need around 3kW from the alternator to run a household aircon and that isn't taking into account the reduced size of the radiator and small size of the pump. 3kW @ 12volt isn't going to happen.