I used lacquer thinner, but I was only doing one. If you were in business for yourself, you'd be concerned about the cost of the flush - and everything else!

Adding oil is easy - wherever is convenient! Seriously, so long as there is some oil in the compressor, you can add fresh oil anywhere else in the system - it will migrate through the system when it's charged & running.

On my 124 car the most convenient location was where the suction side of the manifold hose assembly connects to the hard tubing from the expansion valve. It's up high on the drivers' side wheel well. Just unscrew the coupling using two big wrenches. I stole one of my wife's measuring cups from the kitchen (Shhhh...don't tell her) and just poured the refrigerant oil down the manifold hose toward the compressor.

If you're working on a 123, doesn't the suction side run parallel to and just behind the top of the radiator? It should be real easy to disconnect this and add some oil. Now, if it's been a while since that particular junction was apart, you should replace the o-ring. Be sure to lubricate the o-ring with refrigerant oil before installing - they don't work too well when they get pinched.