No offense, but it does sound like you should have your AC system professionally serviced.

When you ask about the "accumulator," I assume you are referring to the dryer. The dryer is not just a tank to collect water, it contains desiccant to remove water from the coolant, it does need to be replaced and should not be exposed to the atmosphere or an uncharged system.

The expansion valve provides the pressure drop between the high and low pressure portions of the system. Rust is not an issue, but these valves can get clogged if there is any debris in the system (i.e., from a failed compressor). After having a clogged valve take out a new compressor, I now insist on having them replaced when I replace the compressor (they are cheap, maybe $50).