The expansion valve is a block at the evaporator inlet/outlet basically underneath/behind the glove box. A flush gun works very good and does indeed cost about $50. I would try to find a local automotive air conditioning supply. Most communities have someone who does mobile AC repair AND sells parts. They usually are a good source of advice and parts/supplies that you need.

I would recommend removing the compressor, disconnecting all lines including; condensor, evaporator (this is diconnected when you remove the expansion valve) and the filter drier at the right front corner of the engine compartment. Fill your flush gun with flush and blow some flush through each unit, then blow it out with compressed air. Put in the new filter/drier, use green o-rings and nylog o-ring lubricant on them. Pour out as much oil as you can get from the NEW compressor. Prop it up in a pan and let the oil run out while you are doing all the flushing. Then pour in the correct amount (I think it is 6 oz.) of Ester oil, bolt the manifold in place on the compressor BEFORE securing the lines to brackets above the compressor. Then evacuate for at least 30 minutes with a good vacuum pump, but a longer time of evacuation is better. Then charge system with r134.

What I would consider even better would be to follow above steps except use mineral oil and R12. If you do this you should put UV dye in when you charge so that if a leak remains, you can find it when you have to recharge.

R12 has come down in cost and would give you a much better air conditioner.

I would suggest going to aircondition.com and surfing around in there. One of their advertisers has a good book on automotive refrigeration that would be well worth the $15 or so.

Best of luck,