I don't think you are going to see much value for money from them. Probably the only thing is to leave them on float and hope for the best.
Removing the acid is complicated and messy and even if you do it properly for the first time you will likely not be able to repeat it as they get older. I suspect that with older cells the crap and crud on the bottom will get trapped between the plates when you empty it.
I have a feeling that deep cycle batteries are wasted for this application, I think you may find it more cost effective to use cheap starter batteries and flog them to death and replace them when next needed.
These things have a life, no matter how much you use them they die with age and considering the original high cost I can't see you having much use from them.
Flux