compressor "should" drop in best i can see. replace the accumulator (the black cansiter thing the lines go to up on the firewall) but dont install it till your ready for the system to be vac down and re-charged. id probably even wait till the thing passes a vac check and pressure check just to be safe. There is dessicant in them and once open to atmosphere, it starts absorbing moisture. in a high moisture state, it would take about an hour or so before the dessicant is no good anymore. Id also get a new orifice tube. wouldnt hurt to repalce all the o-rings and seals you can as well (every joint will have one).
from there, lots of brake clean to back flush the system. spray a shit load of it down the pipes in the reverse direction of flow, then put an air hose to it and blow it all out. i like to put a porous rag on the end of the line to see what kind of crap im spraying out and if its clean or not. i keep filling and blowing out till i see it come out clean. remove the accumulator and orifice tube when you do this. you are just cleaning the lines, evap and condenser.
When you take it to the place that will be filling it (unless your doing it), bring them the orifice tube, accumulator and tell them the system has been completely flushed so it will need the appropriate amount of oil added back to the system. Ask if they wouldnt mind vac testing and pressure testing prior to the new accumulator install as you dont know how long its been apart and could have leaky hoses or seals. they should understand.
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