Always a good idea to flush unless you know for sure the system is spotless on the inside, especially if the compressor cratered or you spot grime inside the pipes anywhere the system was opened. Flush kits that work on shop air (around 100 psi) are not so costly if you buy a bare bones kit where you add the hoses, fittings, and clamps yourself. The flush liquid is overpriced, so when you run it through a big part of the system like the condenser, go slow on the trigger to be sure you don't run out of flush liquid before the job's finished. Evacuating the system with a vacuum pump vaporizes any remaining flush liquid due to the ultra low pressure and pulls it out.
I vote for R12 oil in an R12 system, as per the FSM.
