Hi Greg,
Start with a partial first of around one kg or less. I've just taken about one quarter of a red pill. By doing 25% of the batch as AG you improve the end result, hopefully, but retain the option of tipping the few litres of wort down the sink if you totally stuff it up. Also I believe best results are found by bringing the grains and water UP from a lower temperature gradually rater than simply chucking it all in an esky and letting it soak.
Also, you need to give some thought to rapid cooling when you drain the esky, and about guarding against oxidation when hot.
If it were as easy as chucking it into an esky of hot water and great beer comes out, we'd all be doing it an no-one would be able to sell all those kits. get a good brewing textbook and start learning!
I'm now up to about one and a half kg of grain, done about 5 brews now and feel each one is better than the previous as I refine my techniques. I reckon launching straight ibnto a 4 to 5 kg grain batch is asking for trouble unless you have someone to show you all the tricks.