what toyvo said... the fill fittings are different between 134s and r12... r12 can still be purchased and is a more effecient system, but youll pay more for it. upgrade kits can be bought at your FLAPS for $20 - $40 IIRC.
to clean an old system they will vacuum it out and they will also be able to detect any leaks. be aware theres a good chance the hoses will need replaced, its a good idea to do so anyways and you can expect to pay $100 for the hoses. I would reccomend you install all the components and then go to your a/c shop and have hoses made to fit perfectly.
also may as well purchase a new drier they are cheap and it wont smell bad like a used one probably will.
good luck!
