Hmm, your description reminds me of an old Garrard type A my family had when I was a kid, except it had a heavy arm. The gears and levers underneath were a marvel of engineering. That was a changer though. Anyway, you might be able to get it going with very little effort and money. I'd get your co-worker to join VE and print out the service manual anyway.
Have you taken off the bottom? If it's got gears/levers etc, chances are it has dried up grease that's responsible for the erratic behavior. If so, this must be removed and replaced with white lithium grease. You can get this at an auto parts store for a few bucks. Depending on the mechanism(s) you might have to take it apart or parts of it. That's where the service manual comes in handy. It usually has exploded views of the mechanicals. The idler wheel will have a mechanism also. I don't know a better source for the rubber wheel if it needs one. But if it's running now you're way ahead of the game. If the dried grease is bad you might have to soak the parts in solvent, alcohol or gasoline.
Yea, the arm looks light but it was hard to tell for sure. If that cart is working but only needs a stylus, I doubt if you could do much better for under $40.
http://www.lpgear.com/product/ADCSXLMI.htmlThey also have a MKII and III for $39. They sell Jico so it should be good. I also suggest you use contact cleaner on the pins and electrical connections.