i have done very little a/c work, but worked in a shop which had a bit of a/c traffic. i'll relate what little knowledge i may have gathered, hopefully accurately.
like bweber, we seemed to have quite a few cooling complaints from r134a customers, whether their car came with it, or it was retrofitted. it just is not quite as good of a refridgerant, especially if the car is not moving.
when you retrofit an older system, sometimes you need to change the hoses too, because the oil soaks into them, and r134a and r12 oils are incompatable. the hoses start to fall apart, and destroy the system with particulates, or leak.
there are other refridgerants besides r12 which you can put into an r12 system. the most common is brand named Duracool. check on local and state laws before you do. it is basically lighter fluid, which sounds scary, but it is no more dangerous than other refridgerants, a fuel tank, or a battery. your windshield wiper fluid is highly flammable too.
but the answer to question #3 would be 'yes' it can be that simple. but you should also have a sticker stating which type of refidgerant is in the system.
i dont know specifically about the muffler and dehydrator, but i believe the dehydrator should be replaced with a new part if it has been open to the atmosphere. at least that is what i've been told about receiver/dryers. perhaps the dehydrator is serviceable. the whole system should also be drawn under vacuum for several minutes to help remove moisture.
if you do switch to r134a, it cools better with the system not filled to capacity. i am not sure, but i think 80% or thereabouts gives the best cooling on a retrofit. like if you have a 2lb r12 system, you would only put in 1lb, 12oz of r134a and oil.
your mechanic should know most or all of this stuff anyway, because most states make you take an epa test just to handle freon for buying and selling, and then for servicing, you need to be certified anymore. the certification tests include all the info+ which i just related, and more accurately, i'm sure.
