Hi,
One of the main choices involved is what kind of battery to use.
NiCd is hardier than NiMH. NiMH really requires a special charger that can detect when the cell is completely charged.
Another good choice is Li-ion, but again there you have to charge with a charger that is made just for Li-ion.
Lead Acid is more of a pain than anything else.
Simple Li-ion chargers can be made from cheap LM317's. NiMH chargers really need a special charging chip. NiCd doesnt require anything too special, maybe just a current limiting resistor.