I have a fondness for old minis too.
As well as a fondness for new ones...
I am not sure if Lead Rot is a real phenomenon or not. Apparently, on some of the older completely lead figs, it is an area of corrosion that sometimes leads to eating up an entire section of the mini.
I have had lots of minis and I think that one time I noticed a \'wierd spot\' on an older one. So I washed it thoroughly with soap and a tooth brush. The \'spot\' seemed to be only on the surface, and came right off.
I am pretty sure though, that if the lead rot is real, a coat of primer should seal the minis and protect them from it.