Do a search here - there's a lot of discussion about conversion. You could read for hours, no, for days on end. Here are the basics:

* R12 is not scarce and is not unjustifiably expensive.
* conversion will require that every single component of the A/C system be disassembled, flushed completely, reassembled (preferably with new o-rings), and then recharged with the correct oil and refrigerant. For best results, expansion valve should be replaced with one specifically designed for use with R12. Sometimes this can be hard to find because manufacturers don't typically make R134a expansion valves for cars not originally designed to use it.
* conversion will result in a system that performs less efficiently than if you just leave everything the way it is. On a 95 degree day, my R12 car puts out 38 degrees at the vent whereas my R134a converted car puts out 48 degrees, and can slowly climb to 58 degrees when the vehicle is not moving. Depending on where you live, you might not care too much about this. Installing a P-flow condensor can help.
* conversion will put a much higher load on your compressor; a typical R12 system runs at about 200 to 225 psi on the high side, whereas a converted R134a system might run about 350 to 375 - premature compressor failures (especially with rebuilt units) are not at all uncommon.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280