Before adding freon, I would suggest you hook up the system to a manifold and determine both high side and low side pressures. There are several areas that could be preventing the system from cooling effectively, adding freon could make matters worse. As much as I dont buy tools from harbor freight, my $50 investment in a manifold and $100 investment in a vaccuum pump have paid for themselves many times over.
Regarding type of freon - do not use freon with any kind of sealer in it. I personally use the cheap $5 a can freon from Walmart with no sealer. However, I typically put a vaccuum on the system (after having the freon captured) to determine if it has any leaks / boil off any water in the system, add the correct type and amount of PAG oil and the correct weight of freon.