If you know you're sticking with R12 (a fine choice, IMHO), the best compressor oil is tried-and-true mineral oil. If your new compressor comes filled, that's almost certainly what it comes filled with.
To answer your question about whether you need to take oil out, oil tends to get distributed throughout an a/c system, with portions of it residing in the compressor, condenser, evaporator, and receiver. If the system takes, for example, 8oz of oil, you can estimate rule of thumb 2oz in each of these four components. The ONLY way to be 100% certain how much oil is left -- and thus how much to add -- is to flush the entire system. Because there's not an a/c oil dipstick, over time a/c systems either have too little oil in them or too much (many mechanics routinely add 2oz of oil whenever the system is serviced). But if you're replacing the compressor, condenser, and receiver, all that's left is the evaporator; You could take 1/4 of the oil out of the compressor, but I wouldn't worry about it too much. I'd just leave it be, but I'd remember that you could be slightly on the high side.
If you're rejuvenating a long-dead system, you should flush the system anyway, which flushes out any crap along with whatever oil is left. Flushing is achieved with a canister that you fill with a/c flush or mineral spirits, and pressurize with your compressor. Sounds like you're replacing the condenser, so flushing that part is not necessary. Ideally you should flush the condenser, evaporator, and every hose. You should never flush a compressor; you should just fill and drain it a few times. And the dryer should just be replaced. Actually you can't flush through the evaporator because the expansion valve is in the way, but you can pop-flush (hold the flush canister on the evaporator input, then on the output, and have it build up pressure until it pops off, then use the same pop-off technique with compressed air).
All that having been said, there is risk in flushing because there is risk in taking off hose connections that haven't been disassembled in 35 years. If a car has been sitting with the a/c open (ie, no compressor, hoses hanging), or if the compressor has seized and may have thrown metal into the system, you do need to flush, but if you're, say, replacing a leaky condenser, you can risk going without the flush.
Back to the oil... Last year I bought a '93 Toyota Land Cruiser with non-functional a/c. I flushed every line. I leak-tested it (I use pressurized nitrogen) and found a leaky condenser and replaced it. Then I leak-tested it again and found a leaky evaporator core and replaced it. The old core was wet with oil. I leak-tested the system a third time and it held. I was about to evacuate it and charge it up when I thought... gee, with that core wet with oil. I wonder how much is left? The only way to know was to pull the compressor and turn it upside down. I'm normally not a do-it-once-do-it-right guy, but I sighed and pulled the compressor. It was bone-dry. No oil at all. Had I not done this I would've cooked the system.
Piece of cake compared to stabing that 4 speed while on your back.