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The 2000-2006 and the 2006 onwards vans came with air conditioning as an option (standard on the Tourneo), and as such it can be retro-fitted to vans without air con, however, the cost is great.
You would need to obtain the complete system, new or used, everything down to the relays and the switches, then have it all installed, which means pulling the engine bay apart, fitting pipework, pumps, radiators ect ect, as well as doing the same with the interior, including a new heater box and control unit. You would also need to tell the ECU that the air con con is there so that it can communicate with it and alter the engine operations accordingly, this needs to be done with the correct software on a decent diagnostic computer.
The job in whole is likely to cost between £1500-2000, so you really need to decide its worth, or wether to just trade the van in for a model with air con already fitted.
If the van is refrigerated, you cannot retro fit air con, as the space in the engine bay for the pump is already allocated to the pump for the fridge.
Hope this helps, if you need more info, please let me know.