The adventage is, a lot of FD bodies were designed after the mercury battery revolution - and the one notable that was not is the EF - which stands out by virtue of having a voltage compensating circuit built in.

As far as lenses go, I just had a chance to have a close look at a 20mm f2.8 SSC - and, wow, what a lens! To have such a straight image at such a short focal is truly remarkable! It is not very fast - but quick enough, and I have seen them go for less than $150 on eBay. If you need or desiresuch a WA lens, this one is definitely worth a look!