Geoff
I second the idea sent by slingshot.
testing them before paying would be the best bet. I have a few dozen taken out of old UPS units and they are good for short runs such as test loads for new gennies, emergency lights made out of LEDs and just general usage while others are in the dark.
One thing, if you can build a small de-sulfanator and run it on these for awhile to get them back up to working levels, then make sure and have a low volatge cutoff system that will not allow them to get used too far down. Being that they are SLAs they can be hurt just like a car battery.
If nothing else grap a bunch and have fun learning RE with them.
Cheers
Bruce S