Before just buying stuff I'd do a quick test first; it'll basically just cost you the price of a belt. I once owned a Spider with a perfectly good, fully charged A/C system and no belt, most likely because someone was in a hurry to replace the alternator belt and just cut the A/C belt to get it out of the way.
- Install belt. Remove radiator or skin from knuckles, your choice.
- Turn on A/C with engine running, see if compressor clutch engages (probably not)
- Jump 12V directly to the compressor wire, see if clutch engages. Do this only as long as necessary.
If the clutch engages when forced you likely have no / not enough refrigerant in the system, and the low pressure switch prevents the A/C from engaging. If it doesn't engage, or if the compressor tries to smoke the belt or makes horrible noises, it's new compressor time.
Assuming the compressor tests out OK I've been known to "accidentally" bump the Schrader valve on the refrigerant port to see if there's any freon at all in the system. If I get a little hiss I'll hook up my A/C gauges (you have a set, right?) and try adding a bit of R12 or something compatible, ideally with a bit of U/V dye so I can see where things are leaking out with a blacklight. Often the leak is so slow that just giving it a hit of Freon will keep things working for quite a while.
If the system is completely empty hook up a vacuum pump (you have one of those too, right?) and see if it will hold a vacuum for a bit. If it won't hold a vacuum you should be able to hear a hiss where the vacuum is leaking. It's usually from a fitting or a pinhole in the condenser, etc.
AFTER you know exactly what's good / bad in the system you can decide what you need to buy, whether or not to re-do hoses, change over to 134a, etc.
1989 Spider driver
1974 Spider restoration project