Battery isolators

Hi guys just hoping someone can give me advice on isolators. What size should I be looking at? Is 100a enough? And how important is it to have a switch over type for linking the batteries for starting if the main one fails.
Also any suggestions on where I can get a tray for the second batt?
I'm thinking of putting a sidewinder unit under the bonnet and a manual switch in the car to link if needed probably go with a optima yellow top as the second battery.
Cheers

100A is fairly standard for an isolator, I have an ABR under the hood which came with an over-ride switch on the dash. I have used them before without an issue but the current 1 isn't connected fully until I get a chance to remove my manual isolator which is a pain.

Best members to ask would be Yendor or Evil Twin.

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