The A/C systems on the w108 cars are fairly simple, as there are no high and low pressure switches in the system. The compressor on the 4.5 cars is the old GM A6 type and is located on the lower drivers side of the engine. The dryer is a long cylindrical type located on the passenger side of the condenser. You should see it when you open the hood. This sight glass is located on the bottom of the dryer on liquid line leaving the dryer. The expansion valve is located inside the evaporator box under the dash. If the expansion valve has to be replaced, then evaporator box has to be removed to gain access to it. The amount of work involved in pulling the evaporator box out it pretty high. I've never seen an evaporator go bad in a w108, but i've seen plenty of clogged/stuck expansion valves.

As long as the compressor clutch is coming on when you turn on the A/C, then the most probably cause of poor cooling are low charge and/or a clogged/stuck expansion valve. The easiest way of determining this is to connect a set of A/C gauges and see where the high and low pressures are. Also a worn out engine fan clutch will cause poor a/c cooling and overheating at idle. If the compressor clutch isn't coming on, then check the fuses in the auxiliary fuse boxes along the firewall.

Low high and low low pressures are a good indication of low charge

High high side and low low side is a good sign of a clogged or stuck closed expansion valve.

One more suggestion. I don't recommend converting W108/109 cars to 134a. These cars have a fairly small stock cooling capacity with R12 and when switched over to 134a, you loose cooling capacity. Since you live in south florida you will definitely notice the difference between R12 and 134a.

John