How much do you want to spend?
That's the only real issue.
For a lawnmower, where it's easy to get at them, and weight is not an issue, I'd probably use Sealed Lead Acid batteries. The 12v 7AH batteries are almost a commodity, and are cheap.
Don't pay over $12 for them.
And stack 'em until you have the voltage and A/H you need.
And if you want to spend a lot, go another way.
Arf