Electric only limits and mods?

newb here, looking to purchase my first hybrid, lots of reading to do.

I have this silly idea of purchasing a used prius and a battery or two from wreckers, modding the electrics to use the 2nd (and 3rd?) battery for extended range. Also modding to enable higher speed in electric only mode. Currently thinking the early models are cheap enough to make this a viable project.

I've seen a few kits, mods discussed, including adding cooling on the controller. so not expecting this to be a plug and play solution, plus some engineering/design/custom build will be required.

Is this feasible? Has anyone done similar? I was looking at converting a standard petrol vehicle to electric, but lazy me thinks this might be a better option.

obviously there are a few different battery sizes, weight issues etc. but hoping to find someone who has looked at this before. Or at least traps to avoid.

Based in Brisbane, Australia. local roadworthy/engineering regs are also an issue. Not impossible.