Honestly, I wish you all the luck in the world. Been there, done that.
The aircraft lacks the range, so you need to consider extra fuel tanks.
Due to weight and balance, you need to position the extra tanks at different locations, which in turn does require a complicated fuel balance system. Embraer does not give you the required load analyses/structural info. The authorities will throw in a couple of wrenches etc.
It is not just putting a luxury cabin in. It will be much more and looking at the second hand market prices, you might consider just to buy a ready made Legacy 600.
