Ok I have read your situation. The oil/grease on compressor and the coupling could be further investigated. DIY method of checking would be using soap bubbles smeared all over those suspect areas but soap bubbles may not work if the system pressure is already low. If your PO had added some UV dye into the refrigerant then the fault could be much easier to trace using UV light (black light). Otherwise I would recommend you to bring the car to the AC shop for them to pump in some refrigerant and use sniffer device to detect leaks.

Leaks from coupling could be from old O rings which may be easily replaced. If it's a crimped type of coupling then replacement of entire hose is the easier option. For leaks from compressor, the compressor must be dismantled out of car and carefully inspected where the leak is.

Here is a Valeo manual on compressor repair. May it be of help:
http://www.valeo-compressors.com/media/brochures/TM08-16_servicemanual.pdf

The Diesel-Kiki compressor most used in Volvo would be the DKS 15CH model. The Valeo TM15 (found in the manual above) resembles the same model as your Diesel-Kiki.

If your system has been using R12 Freon then it may be better to continue using that refrigerant if it is still locally available to you. Conversion to R134a refrigerant may need further work of replacing the compressor, all rubber hoses and O rings in your system to be R134a compatible. Otherwise slow leak may develop thru rubber parts over the years.

Yes you could continue to use your R12 compressor in R134a conversion. But the fine print is R134a system deals with higher pressures and could stress your already old and tired R12 compressor. Eventually you'd need a new compressor.

Regards,
Amarin.