Geof, like yourself, I'm an accomplished DIYer that was stymied by a/c systems until I started studying the posts on aircondition.com. LarryBible is a frequent poster on that site.
I agree with Larry that you should disconnect the system and flush all the components that are flushable. You need a source of shop air. Dress the end of the air gun so you can make a seal by pressing it against the openings of the a/c plumbing. I toss some solvent into each component, then blow it out the other end. Repeat this many times. You want to get any crud out that can ruin your new compressor. You also want to get out most of the old oil. You can start with mineral spirits, but you have to finish with lacquer thinner - or better yet - brake cleaner.
Once it's clean, now is a good time to replace all the seals. The dryer must be replaced.
The correct amount of oil should be distributed throughout the system for good startup once you're finished.... a couple ounces here and there... in the condenser, evaporator, receiver/dryer, and about half the oil in the compressor. I don't know what the specs are for capacities, but keep looking.
After charging with oil, button the system back up and pull a vacuum....the longer the better.
Static charge with one can of refrigerant, then start 'er up and finish charging to proper pressures and vent temps.
For further information, you should post on aircondition.com. Also check out acsource.com, ackits.com, and caawparts.com. I buy all my parts by mailorder through these guys. They are better and cheaper than what you can buy locally.
95 E320 Cabriolet, 131K