I like 100 as it's a good medium viscosity oil. Yes, put a capsule of dye into the system. Replace all "O" rings you have taken apart and it's wouldn't be a bad idea to renew the seal a the lineset where it attaches to the compressor and receiver/dryer assemblies too. These sealing rings wear/age over time.
If the system has been setting idle for an extended time I would replace the drier assembly too. That is the black cannister looking thing in the system under the hood. The evaporator core in the dash would be quite easy to remove to physically clean it with the system dumped and this would greatly help your comfort level once the system is back operational.
I would dump two ounces of PAG oil into the receiver/drier's inlet, button the system up, pull a vacuum on the system allowing it to dwell for an hour, (or more) to thoroughly evacuate any moisture present, then pressurize the system to 50psi with nitrogen administered with your service manifold set. Allow this charge to dwell in the system for an hour and check for any leakdown of pressure to ensure no leaks. If good, evacuate the system again with your vacuum pump and charge according to a vapor pressure charge reflecting the ambient temperature and the refrigerant you are using. System temperatures and pressures change with ambient temperature but ensure you have good airflow over the condenser coil in front of the radiator. A box, or tubeaxial fan is what I use and allow that system to operate in excess of 1/2 hour after you think it's good for stabilization. I hate "comebacks" because I got in a hurry.....