Do you truly need such a home? Building smaller will cost less now and you will continue to save with lower taxes, lower utility bills and lower maintenance cost.

That said, spend money on the things you interact with -- things you touch and use.

Are you ever going to sell the home? I would spend more money on the major things that won't have to be replaced any time soon. Money should definitely be spent on the layout of the home, and things that will be difficult to change later on (like flooring). You want to get it right the first time because it will be a hassle to change. Kitchen layouts and materials, bathroom layouts and materials, etc. These areas will cost a lot to renovate and should be done as perfectly as possible the first time around.

Splurge on things like the following-
Windows-go triple pane if possible=energy savings
Exterior finishes=low maintenance
Insulation and air sealing=energy savings and comfort
Quality interior products=low maintenance

Zoned heating and cooling will save you money in the long run. Good quality faucets and bath fittings with parts readily available are much better than trendy fixtures you can't get parts for in five years.

Read a copy of the home warranty.