I dont feel any of it is real heavy. Condensors are as thick as an oil cooler (20mm), The bigger the surface area, the better cooling, most come with mountings for receiver driers on the sides of them. Compressors are sized for motors, so using one off a 2litre motor on a A series would be putting pointless load on the motor. A compressor sized for a 500cc - 1400cc motor, is a little bit bigger than an EA ford 80amp alternator. Using a smaller compressor on a bigger motor is not a problem. The bigger the cabin area, the more gas you want to compress quicker, thats when you want a bigger compressor, Just like house A/C systems. You can mount the compressor on any side of the motor aslong as you (a) make the mounting, and (b) line it up with a pulley so the belt runs smooth.
The evaporater works opposite to a radiator, Where as it cools the air blowing through it and removes the moisture (humidity) from the cabin. This moisture is dumped through a drain tube to underneath the car. Any auto elec that does A/C will have a catalogue you can look at which has dimensions of aftermarket underdash units.You can still buy the similar style units as to what nissan dealers fitted to 1200s.
You can easily fit up the components yourself. The underdash units require basic wiring ( easier than a radio) and they come with diagrams.
You can buy the rubber high and low pressure lines buy the metre from pirtek etc, cut it to length and then get the correct fittings crimped on buy the same shop etc. Then oil and gas at the auto elec etc. Most aftermarket fitouts use a rpm trigger which cuts out the compressor below a set engine rpm, this makes engine idle not difficult at all.