I build battrie packs for my model airplanes all the time. The mAh rating is for the cell, add the individual cell voltage togather in series to get pack voltage, but the Ahr rating stays the same. You can get subC cells that go to around 2000 mAh, and D cells that go higher. If you want to go the route of NiMh, I suggest you get a charger designed to a delta peak sensor, or heat sensor for charging. Learn how to program a charger, charge rate X cell +the Delta per cell you want your cut off, and you can expect packs to last a long time.
One other thing I might add. NiMh and NiCad can be fast charger. Lead acid can not. NiCads are more forgiving and have a wider fudge factor than NiMh, but NiMh are lighter. If you want to use a pack type holder I think you can buy a battrie charger at Radio Shack and take the individual cells out of the holder and charge as individuals. You could try and find a source for a 6V individual SLA battrie, like a cam corder battrie, and use this. Instead of leaving the walwart charger pluged in all the time. Set up a light timer on an extension cord so it only comes on for say 1 hr a day after the initial charge. In other words, set up a 24 hr timer to come on once a day, for one hour. This will keep your pack ready to go, and not dammage your pack by over charging.