Get a new filter/drier when doing this job. Its suppossed to be changed anytime the system is opened and exposed to air. The drier is a hydroscopic bag that absorbs moisture out of the system, if the system is opened up, the bag will absorb humidity out of the air and fill up, thus you should get a new one.
As well, the PAG oil used in R-134a systems is hydroscopic and will absorb moisture contaminating it. I'd seal up the compressor well, until the install, drain/flush as much of the oil as you can. Make sure you flush all the oil out of the other parts. That way when you service it, you just add the full capacity of oil, its the only way to make sure you have enough oil.
Too much oil takes away refrigerant and lower the cooling capacity, too little oil will let the compressor burn up, so more oil is better than less, but you want to be as close as possible to the proper amount of oil. Fresh PAG oil would be best, since any of the old oil coming from the system that was broken open and removed is most likely contaminated with moisture.
As well, if you don't have a good vacuum pump/source, I'd take the vehicle to a Professional to be serviced once its put together. Without a proper vacuuming down, there will still be air and moisture in the system, the air and moisture will form acids in the systems and destroy it. It is vital to have the system hermetically sealed, with nothing but refrigerant and refrigerant oil in it, no air or moisture. The new filter drier should absorb any tiny trace of air/moisture that remains after a proper vacuuming down and waiting 30 minutes for remaining moisture to boil out while at a vacuum.