I just bought a used Juno 60 with some problems. I was able to fix it up without much effort but over the course here's what I discovered. Please note: I'm no expert on this or any other synth. However the service manual is clear enough that I could track down my problems with just the block diagrams. Very useful.
Sliders are near impossible to find but they can be rebuilt. They have to be desoldered, disassembled and cleaned. That would probably be a good thing to do anyway.
Pots can be replaced. The bender pot might be a problem to find, I need one but haven't looked. Buttons and switches can be replaced as well.
Test the envelope. If any of the voices have problems with the envelope it could be a bad envelope chip. Those are custom Roland, hard to find, and run about $30-$40 a pop.
Many other controller chips can fail but there are usually off the shelf replacements for a few bucks. In my case the envelope wasn't working with the VCF. Turned out to be a controller chip that cost $3 + a socket to replace.
I think VCF chips are custom Roland as are VCA. If you spot problems with those expect to pay premium prices for replacements.
Finally, I installed Minerva for the 60. I think it was a wise choice. Adds MIDI, a 2nd LFO and a dedicated ENV for the filter. I think that upgrade makes the 60 the most desirable Juno because of the extra modulation and added features. And it's reasonably cheap too.
That's all I know, good luck!
Edit: It's a fun synth. Very simple, but that's a large part of its charm for me. Just about every sound out of it is good to me. It sounds better to me than my Jupiter 6 and Polysix, though not as flexible as either of them.