A normal car battery will work, but the gel batteries like that seem to handle deeper discharges if I remember right, they also tend to be easy to find a direct replacement for the battery.
On my electric lawn mower I replaced the gel batteries a few years ago and they are still working ok, 4-5 years of use is common, but when they are in need of replacement next I plan to go with Lithium, I haven't found anything much cheaper that looks better then
PingBattery LiFePO4 Battery packs for Electric Bicycle e-Bike & Scooter, Lithium Iron Phosphate Battery but he doesn't have any 12v packs, a lot of other mowers are 24v or 36v.