I think the real trick to any Battery Pack is to make sure the Batteries are charged when the pack is not in use. How many times has one grabbed a battery pack only to find that the last guy that used it didn’t plug the charger in.
I like 8D Batteries too. I know that they are big and heavy but they store lots of potential current flow.
So if you want a great Jump start kit for under $500.00 I would build my own out of 2 or 4 8D batteries, a cart, some made up cables and a battery charger to replace the used current.
If your crafty enough you can even set the thing up for 12 or 24 volts