We just completed a 2200ft² single story over basement house last year. Our thought process was very similar to yours (grow-old-in-it potential, energy efficient, simple) so feel free to PM if I can help with more specifics. Some quick notes based on some of your questions thus far:
1) BIBS is what we chose for wall cavity insulation. I was very impressed with that system and especially the noise dampening. Fills in behind outlets, switches and dryer boxes perfectly, which is something batts will never be able to do. My advice: avoid batts all together if you can. It's too hard to find someone who can do a proper installation even though it seems so simple. They must be cut perfectly without compression. Tricky to do well. Spray foam in wall cavities is just not cost effective. And blow that fluffy stuff in thick in your attic and use raised heel trusses.
2) We went with 2x6 exterior walls for the added rigidity, extra R value and deeper window openings, which look better and accommodate blinds easier. For sheathing we went with Zip-R in the 1" overall thickness, which is a half-inch of Zip panel + half-inch of foam board bonded to it. So you get R-3, which isn't much but it's in the most effective location where it can eliminate the thermal bridges at the studs and other framing members. The foam is on the inside so you still have an easy nail base so your siding contractors won't have a fit. At 1" overall it was easy to order windows with correct jamb extensions so custom work on site was not an issue. And the taped seams of the Zip system provide excellent air sealing. We also used Zip panels on the roof, but without the added foam board.
3) Do you have the slope to do a walk-out basement rather than a crawl space? If not, then an encapsulated crawl space is definitely the only way to go. Might even need a dehumidifier down there. Just priced encapsulation of an existing crawl for a family member and 5-6k is the reasonable range to get it done right. Good read here:
Building an Unvented Crawl Space BTW, that is a great site with good info on all the considerations you've mentioned. I read that site, and elsewhere, for nearly 5 years before we started our project.
4) Our house was built to Energy Star v3.0 and Indoor Air Plus standards, which I would highly recommend looking into. Having an energy rater as a 3rd party, extra set of eyes is a huge advantage and with energy modelling software you can evaluate different options and how they will affect your overall energy usage. A blower door test should be standard for all new homes. Just remember that a house built to Building Code standards is not some lofty goal. It is simply the worst house you can legally build without facing fines or jail time. So you're definitely on the right path in trying to exceed Code minimums.
Also, make sure your HVAC guy is capable of doing proper load calculations. The last thing you want is a rule of thumb sized heat pump. He should also be able to layout your ductwork in the most efficient way, which will allow you to order trusses with duct chases (square openings) for the main distribution ducts.
5) For the universal design bit: door openings of 36", no-step entry at least from the garage into house, 48" hallways, lever door handles, rocker switches, curbless shower and outlets 18" off floor. Those are some of things we made sure to do. Will add more if I can think of them. But the most important aspect is energy efficiency. You spend more upfront to ensure that the home is cheap to operate over the long haul. That's what you will appreciate most when you hit retirement and your income is more limited.
6) Insulation is important, but I can't stress the benefits of air sealing enough. "Build it tight, ventilate right." Mechanical ventilation using an HRV/ERV, with your HVAC guy designing that distribution network too, because after all, that is what the V stands for. We used a Venmar ECM ERV, which are now badged as Broan. We have it running 90% of the time, either bringing in fresh air or on recirc, and it only draws the equivalent of 2 cfl bulbs. Basically no energy penalty in exchange for a huge bump in indoor air quality.
Hope that helps.
