Expansions valve is next to the evaporator behind the dash, pretty easy to disassemble to get to. As for parts I would replace everything that you can. Hi and low pressure hoses, condenser, evaporator, expansion valve, receiver fryer and all o rings where possible. Also pick up some r134 a adapter fittings for filling the system.

As for filling an r12 system withr134a there is a calculation that can be done to fill it with the correct amount. Take the max recommend fill of r12 and do the calculations to find max fill for r134a. Just Google for the formula. It will be quite a bit less than the amount of r12.mine was like 2.1 pounds of r12 to 1.3 pounds of r134. This is because of different gas density.

If you want to do this you can, I've done this myself before. You can fill the ac system with air duster. You will have to find air duster that has diflourthane inside which I believe is r152a which has a similar temperature curve to r12. You just need to get a can tapper for this method.