Being honest, a Zero 100 is a collector's item and a conversation piece and climbing in value steadily. Being mechanical, the greases used back then DO dry out but the mechanism isn't hugely difficult to strip out, clean, re-lube SPARINGLY and re-assemble - if I could do one as a teenager (my first AP76 which has an almost identical mechanism only differing in tiny details and mainly in the auto-trip slider assembly, which uses a finer rod and slightly different cam parts - changed again in the SB models more in line with what Dual were doing), then with adult care and delicacy, you should be able to tackle it and being a later design, you may not need to do so much - I can talk you through it and there's threads and threads on Vinyl Engine about fettling these machines.
As Adam beobloke says, it's actually NOT a pile of crap at all and with careful cartridge choice and a quiet drive, it can still sound very pleasant indeed and hell I'm going to say it, potentially better and quieter than the once ubiquitous Pioneer 12D models which got worse as production was ramped up and evolving into the mk2 version which didn't seem as consistently engineered. Garrard put their all into the basics and did them well.
Go on, have some fun To see one of those things working and sounding just fine once sorted is a wonderful experience I assure you!
Tear down these walls; Cut the ties that held me
Crying out at the top of my voice; Tell me now if you can hear me