The thing with leisure batteries that is more important than even ampere-hours is maximum cycles of charge.
If you are offered a battery which doesn't state that information, don't buy it.
Many leisure batteries will have 150 cycles of charge. That is from 11.8v to fully charged just 150 times and then it is a dead, deceased, battery-no-more battery. Recharge from say 12v to fully charged is half a cycle, roughly.
There are batteries that will give 1000 cycles but the best you will usually find is 500.
If your battery is really difficult to get at, look for a "sealed lead-acid". They are maintenance free and much better performance than gel or AGM. If there is room, go for semi-traction.