I have done about 6 Kit Beers & I now want to move to AG.

Over the coming weeks will you please all bear with me when I ask question upon question. I like to get things straight in my mind before I embark

1st I have read on this forum about 'efficiency' and then expressed as a % What is this about? Do I need high efficiency or low.
2nd For a mash tun, I seen 2 FV's placed one inside the other with the inside one having 2mm holes drilled in the bottom to let the water Ahem excuse me Liquor through and the outer FV having a tap to draw of the Liquor. Will this work effectively as a MT?
3rd Can I convert an FV to a boiler using something like kettle element ? If so how do I do this? Will the plastic FV stand up to the boiling water temperature?
4th After the boil I have seen Copper Pipe Coolers being used to crash cool the Wort so the yeast can be pitched. To save money can I forgo the Cooper Cooler and just leave the Wort overnight and then pitch the yeast the next morning when the wort has cooled to room temperature ?
5th I think I know the answer to this but given I am going to spend a lot of my hard earned on more equipment I would like some extra some reassurance

Thanks for reading Guys I do appreciate any answers that you post.
Regards FB