If your looking for a pedestrian stacker (walk-behind) then you have no choice but to go electric. The others are right in saying what they have as most second hand electric trucks come complete with a totally shagged battery!. If your looking along the lines of a 1000kg stacker (especially the older Crown kit) then it's probably gonna take 4 (6v) mono-bloc batteries,so price 4 new one's into your spend. The trucks are generally very reliable before the 7-10000 hour mark so less than this and new batts should be spot on for your needs. Charge about once a week to conditon the cells then you should be good for several years!.
Hope that's a help and feel free to ask if I can help anymore!