I'd use the Nygl (sp?) recommened for your application. I'm assuming you are staying R-12? Yes absolutely go with the newer style o-rings (green) when possible.
Flushing-did this car have a bad compressor failure? You can get a general box of green o-rings at harbor frieght for $10.
I've flushed with flushing chemicals and then also used a general clean petro-based product which leaves very little residue. Yes probably more flamable but I've done naptha as it's $10/gallon with success. You could always follow with real flushing solution.
If you've had a bad failure, a screen on the compressor intake would be a good idea from NAPA.
Long evuation time is good. I'd say 2 hrs MIN but 4-12 is better. But, unless you know that the evacuation pump is working well- no leak in hose connections etc.. pretty hard to say. I've got a submicron gauge and I shoot for stability at 200 microns or less. That says nothing is leaking or boiling off still. Definately shut the hose pump connection to the hoses off and it should hold vacuum with no sign of leakage. Check low side reading and wait 1/2-1 hr and it should be the same.
I ussually replace the switches, but that is down time and acess to recovery aquipment. Distribute your total oil charge within the system as an R4 compressor doesn't have a sump.
If you have dry nitrogen- you can use it and leak detect easily. I've used soapy water with glyerin added many times.

M