Rod Bacon wrote,
There's a cheap 5 foot tank on eBay, not far from where I live. It's an
older steel-framed job, on a steel stand....

I like the looks of the older tanks - the older, older tanks, have a
slate bottom. They were sealed with tar - if it leaks all you have to
do is fill it with warm water, it will reseal itself. I would reseal it
with silicone myself.

I'm not
sure what I should be looking at when I inspect this tank...

Already pointed out - rust. I would also look for carcks in the slate
bottom. Around here, those old tanks go for around $ .50 per
gal............ Frank