How do I know when a derailleur needs replaced?

I have this early 90s Trek 1200 that I'm looking to refurbish. It was my former commuter that I inherited from a coworker and it served me pretty well for a few years before it retired when I stopped bike commuting. Now, I'm considering making it a refurb project, since the frame is still in good shape and I'm interested in trying my hand at some mechanics I've not done before. I figure I can spend a few hundred over the course of a couple of months putting it back on the road - not my preferred type of bike but I did not hate riding it.

The chain is definitely worn out and I suspect (but don't really know how to confirm) that the chain rings and cassette are as well - being as a worn chain wears the teeth. This makes me think about the only other part the chain goes through. I don't remember it having any shifting issues - though I used only the friction-shift setting for extra hipster street cred - but if the steel cassette is likely worn, I definitely expect the plastic derailleur jockey wheels to be. This is a Shimano RX100, which is long out of production I think, so I'd have to hunt up a 7-speed unit that actually works with my downtube shifters (which are friction-or-index changable).

When should one replace a derailleur, and is it viable to replace just the jockey wheels if need be?