I have the 500/5.6 lens, is that the one you mean? I am not familiar with the 500/8. Is it a mirror lens?
Anyway the problems you describe sound tolerable. That the ring moves past infinity isn't such a big deal, but if you want near focus, I have always used an extension tube for that anyway. As for the fungus, I do think the front element can be removed but it's probably best done by a professional. To kill the fungus is easy enough... with UV light, but cleaning off what's there may require some careful surgery...
I'll just add, I recall that in viticulture, it is common practice to burn a sulfur stick in the barrel to kill bacteria, mold, and maybe also fungus.
Do bear in mind that the 500/5.6 lens in good condition often goes for ~$600-700.