Air conditioning advice sought - '90 W201

[Greg in Oz]

I purchased my 190E during the summer before last (Jan '03) and noticed that the air conditioning was not very cold. I did not worry about it too much until early last summer when I had it re-gassed (and was told pressures and everything looked fine). It worked great for a couple of months but then its performance began to deteriorate again. It is still running with R12. With summer again approaching, I want to get it sorted. Being someone who does all my own repairs and maintenance (apart from work such as auto tranny rebuilds and air conditioning) means I don't have a known and trusted workshop for this work. A fellow club member does still have some R12 and is prepared to re-gas it for me but is concerned with the slow leak. I asked if he could put dye in the system to locate the leak but he feels the leak is so slow it will be difficult to find. He is also of the opinion that it will probably be the compressor seal and that a new compressor may be required. The car has travelled 226,000km (about 140,000 miles) since new in 1990 and I have no reason to believe the compressor is not the original. My 300TE is the same age but has travelled 184,000km presumably also with the original compressor (both cars have similar Nippondenso compressors but that in the 300TE is slightly larger). The 300TE has had R134a retrofit prior to me purchasing it in 1998. It cools perfectly and I have only had refrigerent added once (several years ago) in the time I have owned it.

My questions are:
1. Is it a fair call that the compressor will be at fault?
2. Can a leak that takes several months to prevent the system working, be detected with dye or by any other means?

I don't want to start throwing lots of parts (and lots of money) at it. I would prefer to only repair whatever is necessary. Obviously I could have an R134a retrofit done, but I am concerned with the cost involved, and I also believe this can be even more prone to leaks due to the higher pressures involved and cooling efficiency can be reduced (although the R134a works great in my 124 wagon). The cost of an R12 re-gas every summer rules that out as an option also.

What do the air conditioning experts among the members here recommend (Larry and others)?