how much trouble to put ac back on?

Hi , you will probably need to flush the evaporator with an approved air con flushing agent , not the sort of job you can do at home - you will need to visit a car air con service person for that. The condensor if left open will also need to be flushed if its still ok and doesnt leak .The drier will need to be replaced and then the system re hosed. The compressor if left open will also be no good and to be safe , replaced with a new or reco unit
Unfortunately none of these jobs are really possible without the correct equipment apart from mounting the compressor , drier and condensor.Just remember to keep everything sealed as much as possible
ben