Encore-
I actually designed and built my room. Not from scratch, it was added as part of a major remodel. Main thing I learned, its easy to say things like the ideal room when in reality there is no such thing. You are onto something when talking about great sums being wasted chasing what was made near impossible by the room itself. You are also onto something with the comment about the soundstage and the room. Yes the room matters but much more with some aspects than others.
Even though you are designing from scratch still you have restrictions of some kind. Physical, aesthetic, whatever. Point is only you know them. So my best advice is find a good architectural book and spend a bit of time studying basic construction.
A little knowledge here goes a long way. Here you will learn for example that 5/8" sheet rock stops sound about twice as good as 1/2", or that the relatively simple construction technique of offset wall studs is good for a good 6 to 10 dB of isolation. A solid core exterior door with weather-stripping is nowhere near recording studio performance, but hugely better than standard interior construction for hardly any more money. Just a very few examples of what you can quickly learn that can get you big results for a very small amount of money.
You can probably tell I'm much more practical as opposed to theoretical oriented. Maybe that's because after having learned the theory and then done the physical I learned the practical is much more relevant. (Watch how those who argue this point have never actually done what I have actually done!) So learn all you can, but keep it real.
With that in mind, size matters. The bigger the better. The bigger the room the fewer the problems with bass response. Bass is the biggie. I can get you outstanding imaging in a closet. I cannot get you outstanding bass response in even a decent sized room. There's simply no substitute for size.
One last word on bass response. Everything else is easy compared to this. Vast sums can be thrown at this problem. You need to decide how important it is to you. If you want really deep, powerful, articulate bass and are building your own room then either you spend almost as much on just this one thing as everything else thrown together, or you design the room to include bass drivers mounted directly in the floor or wall. Few do this because of the obvious- cutting a great big hole in the floor or wall. I didn't do it because I only learned of it after the fact. But Fosgate did this, and SpeakerLab in Seattle, and the bass it delivers simply must be heard to be believed. For a tiny fraction of the cost of doing it any other way.
That's about all I can tell you in broad strokes. The devil is in the details. The more of yours I know the more I can help. Ultimately though nobody will ever be able to help you more than you yourself. (But I would definitely take Michael Green up on his offer!) You're on the right track. Good luck!