This is one view on how to climb Everest: http://www.alastairhumphreys.com/climb-mount-everest-2/
I asked a very similar question to yourself on this very forum about 8 years ago and still haven't climbed Mont Blanc. However, I have climbed many mountains in the Alps, started climbing, gone mountaineering all over the place, and recently tried to summit an unclimbed 6000 m peak in the Karakoram. It's been perhaps the best 'journey' I've ever been on. The bit of advice that made the difference was someone telling me not to get dragged up Mont Blanc by a guide, where you will learn nothing, but instead go on a course that will give you the skills to do it yourself: far more rewarding. I would wholeheartedly recommend that approach, and for that, ISM are about the best in the business.
Well done getting a down bag and I wouldn't worry too much about hydrophobic down or otherwise: untreated down floats on water and requires a serious soaking to make much difference to its warmth.
Unless you cannot afford a down bag or are on a multiday very serious expedition in appalling conditions where drying your bag is impossible, there is no good reason to buy a synthetic bag. Unless your tent's rubbish I suppose.