There is an automotive air conditioning bulletin board that is comparable to this site on MB automobiles. The amateurs and DIYers ask the questions and there are professional techs who provide the answers. Try:
In general, they refer to the inexpensive kits as "death kits" and say their experience shows that somewhere between two weeks and two years after installing one, you will be replacing virtually every a/c component forward of the firewall.
There is a wealth of information on that board about automotive a/c and how to do the conversion from R-12 to R134a correctly; what oils to use, how to flush out the old oil, changing o-rings, etc.
One thing they say is mandatory in making the conversion is replacing the accumulator or receiver/dryer with a R134a compatible unit; otherwise the old dessicant will eventually spread itself throughout the system, ruining the compressor and requiring a lot of work and money to correct.
I would urge anyone interested in doing his own a/c work to spend some time reading that board. You can spend hours gleaning valuable information.
Hope this is helpful.