As long as your old compressor didn't blow up, I don't think you really need to flush the system. If you did want to flush the system, buy a blow gun that hooks to an air compressor and has a hose attachment that sucks up cleaner, and use mineral spirits. I've flushed two cars now and I would not recommend it if you don't really need to do it. Its a really sucky job. I did it because I was converting gases and therefore had to get all the old r12 mineral-based oil out.
As far as what kind of oil to use, I would talk to the people who sold you the compressor. Often a certain oil is required if you want to maintain the warranty on the component. BUT, if you are going to flush the system and make sure all the old oil is out, then the best oil to use is something called Texaco HFC. The guys at www.ackits.com
will sell it to you, and they recommend it in all their applications. Talk to them about compatability if you don't flush the system. These guys are really knowledgable and friendly. They also sell flush fluid, the blow guns I was talking about, and just about everything else you need for a/c.
Read the following post for a happy-ending story to a r-134a conversion:
vent temps with r-134a w123
'84 300D, 173k
'90 300CE, 163k
P.S. If the r-134a conversion was done properly, there should be a blue sticker somewhere in the engine compartment, or on the compressor, indicating what kind of oil was used when the system was converted.