The only thing I would add is that the exception to the rule seems to be the 50 mm 1.4. The older SSC version is by all accounts superior in durability and optical performance, as well as being sharper wide open and even brighter to look through due to the larger front element. Other than that, buy the newest lens in the best shape you can get - within your budget.
EDIT - Sorry, just realized I was not making myself very clear there: my comment refers to new FD vs old FD, not FD vs FL . I always looked at it this way - FD lenses are what I use, but its nice to know the FL's will fit if need be. It seems that the coating technology has come a long way between the FL and FD range - the FL's seem to be quite sharp, etc., but much more prone to flare, some claim contrast issues and colour balance. The flair is the only thing I can attest to myself. And the price difference is not that big, really.
And just to add to that, there are a few legends only available in the old FD mount, like the 35mm/2.0 with the convex radioactive front element (said to be really sharp but just for B&W, because it tends to have a greenish color cast), and some of the early aspherical lenses that preceded the later "L" lenses, but if you're on a budget, these probably don't fit it.
mercury batteries have a constant voltage while alkaline jump all over the place, they make the meters in older cameras very arratic, the solution is either zic air or the 1.6 volt silver oxide batteries with a converter to drop the voltage
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!