R12 is very hard to come by now a days, as its pretty much obselete. As far as I know, only a few A/C specalists keep it around for much older equipment, other than that, people without a license to use it cant buy it.
If the system has a leak, then there is a good chance that all the pre-existing coolant has escaped, weather it was R12 or R134. The condenser is pretty easy to change and can be found rather cheap, Im pretty sure the front bumper has to come off to change it, but thats easier than it sounds.
As for your compressor, you'll have to fix the leak and test the system... the only way to tell if its working is... well, if it works and is not overly loud.
A/C systems are pretty touchy, and if you're not sure what you're doing, have someone do it for you... its really easy to screw something up. If you're handy with tools and know how to tear into things, then you can change the condenser yourself, just have the system checked and filled by a pro.