Sealed Lead Acid Batteries are a great starting point battery and are backward compatible with most pre-existing equipment. Note that in common with all Lead Acid Batteries only 50-60 % of capacity is useable without potentially damaging or shortening the battery life. Batteries can be protected using battery protect devices which disconnect supply at a fixed point. Battery Monitors are also recommended to monitor usage and current state of charge.
Also see AGM Batteries for approximately double the lifespan or Lithium Batteries which can be discharged to 20-25%, have much longer lifespan and can be charged more rapidly.