you need a NEW reciever dryer any time the system is open or depressurized for more than a few minutes. there's a dessicant inside to remove moisture from the AC system. A drop will destroy the system (compresser and other parts) in less than a season. I had the joy of pulling a compresser off a combine that had moisture in it from nothing more than an open hose for a few hours and the compresser was gone. that being said, there's no reason to spend $100 on them... more like <$30 for most cars. Getting one used off another car is bad!
If someone has a compressor from another car, that's fine to get used.
Try
www.rockauto.com.
-compresser $300-450
-Reciever-Dryer for $15-50
-R134a conversion kit: $50 (o-rings, oil, and new fittings)
-in-line filter: $25 (very good idea given the condition of your system)
propane.... works great but it's illegal and basically you'd have to do it yourself because of it. the EPA fines heavily if they find out.