I'm not too sure that changing those parts will fix your AC problem. You might have a problem with your compressor. Whatever you decide to change it is mandatory to use new o-rings. Expansion valve is next to brake booster. It requires 4 o-rings. You'll need a 10mm and 3mm allen. I usually take out top drain and some vacuum lines for easier access. You'll need four more o-rings for rec/drier 2 for lines and 2 for high and low press sensors.
Capacity of freon on these cars is 2.2 some 2.4lbs. There should be a red lablel next to radiator.
Valve cover gasket is a pretty easy DIY job. Just watch out not to pinch anything around (wires) and don't over tighten. You can damage thread or snap the bolts. Also, watch out for alignment of cover.