I've heard that SLA has a relatively short life for some ebikers that use it it hard. Typically, these are guys that buy a $200 2000W motor on amazon or ebay and try to run it at 25 mph on four 40 dollar SLA's . They work well on wheelchairs that are using a lot less power, and I suppose they would work fine for a trike at12 mph and under, plus short range.
I looked at ebikekit.com. Fairly spendy at $850, but it does have a 4mph reverse, and rated for 600 lbs, and they must know what works.