If you want to use a wet lead acid battery, electric golf cart batteries are a better choice than gas engine starter batteries for various tiny vehicles. Starter batteries are designed to give a short, high current pulse. Golf car batteries are designed to give a long continuous discharge.
Gell cells, such as disk drive backup batteries, are maintence free. Do not leak, can be turned (and used) upside down. Gell cells do not use the same kind of charger as a regular lead acid battery, and trying to use such a charger will ruin the gell cell. Use a gell cell charger and the battery should last for about four years. I have seen, and heard stories, of lots of people destroying gell cells by misuse, but this doesn't surprise me. These are probably the same people who hook up their car jumper cables backwards. Gell cells and wet cell lead acid batteries will melt, leak acid, and/or explode if you really try. There is really no reason to use a wet cell battery for a telescope these days. A 10Ah 12V gell cell is pretty small, compared to car battery, but is quite a bit larger and heavier than 8 alkaline D-cells.
Lead acid batteries have problems at very low temperatures, but this isn't much of a problem at low currents. But this can be a problem starting your car at 4AM on the top of a mountain after observing in the snow all night.
Most NiCad D-cells are just C-cells in a bigger case. The Radio Shack Heavy-duty NiCad is an exception, it is good for 4 Ah.
Rechargeable Alkalines do not have as much capacity as non-rechargable ones, but it is pretty close, 80-90%. I think 12Ah is probably a bit optimistic. If you write to the manufacturer they will send you the specs for their batteries. Radio Shack sells a book with the specs of all their batteries, but I don't think they don't have their own rechargable alkalines yet.
Disclaimer: I think most of the stuff at Radio Shack is junk and wouldn't be caught dead with most of it. But their alkaline batteries actually perform as well as anyone else's in comparitive tests and cost less. They are probably manufactured by the Eveready or Duracell people anyway.