so they want 800 grand to replace the A/C and yet they use compressed air to check for leaks - better remove the car from those cartoons of mechanics.
there are some guides on the web on how to change the evap in this model, its just lengthy - you may want to find a technician who can do it by showing him the pictorial steps.
if your compressor is not noisy and not leaking and the system oil charge is not metallic coloured then its probably in good condition, reuse it - there is a drain bolt on this compressor where you remove and add the oil. Its quite easy.
the expansion valve is either made by denso or egelhof in this car, both are available easily and not too costly either, the dryer OE BMW part is made by Hansa - you can choose to use another brand and it wont make a difference if you follow correct service procedure.
If your system is internally clean - your condenser would be OK too, it is replaced when the compressor spits metal into the system when it dies/locks/siezes.
For a good job find a gas supplier and get a bottle of nitrogen along with its regulator, use that to clean the system, check for leaks and flush the lines (use common sense here not to kill yourself). For cleanup get some acetone or CTC. And get a set of proper sized O rings for your application. --- Best is to order an A/C rebuild kit for this car - make life easy. It comes with TXV, dryer, oil and O rings. Fill oil correctly.
When you final check the system - fill it up with 200 psi nitrogen pressure and leave overnight, the system should hold it. Then connect your gauge set, vent the nitrogen and start your vacuum pump and pull a good vacuum. Block the system with pump running and test if the vacuum holds, Finally charge the system with liquid first (engine off) get something in the system that can allow it to kickstart. Then charge it up fully, OR just push the exact charge into the system and start it up.
while looking for parts look up some GM (General motors) A/C parts - they have some unique parts which are installed as filters in the system. They really work.