There was a guy on photo.net who seemed to know something about it. It seems like he told about everything you need to know, except how to build a power supply and how to know how silver is there to begin with.
If that guy were here right now, I'm sure he would agree with Ian that a rechargeable battery would be a good way to go for the hobbyists. Safe with reliable voltage control.
There is a tendency for pH to go down somewhat during desilvering, so if you use a hardening fixer with a fairly low pH (acidic), it's possible you might get precipitates forming. Probably safer to raise the pH a bit (perhaps to about pH => 6 ?), but I don't have any experience with such fixers (so I'm just guessing).
Aside from the loss of sulfite and pH shift, the fixer should not be harmed. Of course, if you want to, you CAN harm it with excess voltage. At any rate, it's generally not worth the trouble to reuse fixer. Disclaimer: not a chemist, electrical engineer, etc, just a guy who has some experience recovering silver.