most people dont bother as the cost is so high... to do the job properly you need to change a helluvalot.. all the a/c bits have to be plumbed in, then you need to change radiator, dash, fans etc etc...
Aircon cars have a different radiator and slots in the bumper for cooling. In my experience if you retrofit aircon you can end up with overheating problems if the cooling system isn't 100%. Best way of achieving that is a new viscous fan and a new radiator
The upside is that if you plan to keep the car a long time then the a/c isn't likely to need work while you have the car. I find about 70% of W124s I see have defective aircon. I normally budget £500 to fix a/c problems so £1700-odd for a new system isn't as outrageous as it sounds
I know Cheshire Classic Benz will retrofit aircon into the W124 series.
This is what they say:
N.B. This car is not fitted with air conditioning, in common with the majority of these cars as they came from the factory. However we have recently secured from the original manufacturer a limited supply of the air conditioning kits which were regularly fitted by Mercedes-Benz dealers to the W124 models when they were new. We can therefore offer a brand new air conditioning system on this car, fully fitted, for the much reduced price of £1,500 + VAT in addition to the purchase price above. The installation is almost indistinguishable from the factory fitted system, and has the bonus of all components (compressor, condensor, receiver/dryer, expansion valve, evaporator core, switches, brackets, valves and piping) being brand new. Please ask for details.
Lots of cars were brought in without a/c and it was retrofitted post-sale. This avoided additional taxation. I think all the retrofits I've seen have been Diavia
You can tell the retrofits by the a/c gas sticker on the slam panel and by the cutouts for the a/c switches on the dash. Factory cars have soft edges to the cutout whereas retrofits have a hard edge