I'm with Tony, go for a deep cycle battery, much better value for money.
Also, be aware that your guide camera & imaging camera will require power via their USB cables which I note, your Pegasus unit does not have...
If you are planning to use a laptop with it all then that's fine, they will draw power from your laptop... which means, you may need to think about how you plan to power your laptop... as the battery drain may mean it doesn't last particularly long on it's own battery...
My 1.41 cents worth...
