The $463 rebuilt Behr from Partsshop came with R12-compatible oil installed so you would not have to drain and refill it.
If you are sure that there is no refrigerant in the system, then you do not need to evac before removing the compressor.
Jeff has an excellent point--the leak may well be between the manifold pipe and the compressor manifold pipe fitting (2 o-rings), or between the compressor and the manifold pipe fitting (4 o-rings), or the speed sensor (1 seal). Mine was leaking at the first place.
If you pull the spray shield off and look closely at the compressor, you may be able to see the dye.
This is not oil-change DIY stuff. From the questions that you're asking, you may wish to locate a good shop - there is a forum on this site that lists them.