The main A/C line is different on a 94, with an electronic connector. It's usually way more expensive than the 91-93 R134a conversion line, too.
Most Ford systems used PAG 46 oil, though some R12 to R134a conversions use Ester instead of PAG oil. You can probably use PAG if you flush the evaporator, condenser, and have a new compressor, lines, and receiver/drier. Amount depends on the system, usually it's something like 8oz in the compressor (but be careful, some new ones already have oil in them), 5 oz in the receiver/drier (again, some already have it), or some amount in the system as a whole that you just split between them.
You should get a R134a switch. Factory air/four seasons makes one just for the 91-93 conversion, with an R12 connector so it works with the aftermarket replacement R12/R134a receiver/drier.
The original systems almost always lose their charge due to the O-ring seals leaking. You may want to use some Nylog O-ring lube on your new system to prevent this (and be sure to ONLY use the green o-rings too, not black rubber ones), or at least lube them with whatever oil you wind up using.
You also may want to get a new compressor, or a rebuilt from a 94. There are some reports that the stock compressor on the 91-93's aren't strong enough to deal with the pressures of R134a. They are ~$200 new, but it's cheaper than replacing the entire system and redoing everything due to a blown compressor.