VRLA batteries have a very limited life. Even in UPS applications where deep cycles are exceedingly rare, 3-5 years is typical, and if you lose just one cell, you lose the entire string. Wet-cell lead acid will last longer, but they'll need to go around every month and check electrolyte levels and do other preventative maintenance. And they might not like the idea of all that acid in their basement.
Li-Ion battery has made HUGE progress in the last few years, and may be worth considering. They've developed techniques to stretch the life out of the batteries, too. If I recall, the Chevy Volt only charges it's Li battery from 40-80% capacity, never fully charging it or discharging it. And Chevy is expecting it to last 10 years under normal daily use.
*All* these batteries need to be air conditioned, too. VRLA battery life is cut in half for every 15 degrees above 77F, and others are similar.
Of course, if your client is up on solar as much as it sounds, they probably know *exactly* what they want and all the pros and cons of it and just need you to put their plans into action
