For me, treated posts set in the ground with concrete is still the best way to build a pole barn. With proper drainage so the posts never get wet, it will be there for 100 years without any issues. Creative Marketing has led to a large variety of ways to spend more money accomplishing the same thing without actually making the building stronger or last longer.
I've never dealt with this scenario but the thing I like about concrete in ground with posts bolted to it is that if you ever need to repair/replace the post it should be doable without much disruption to the building. You can support around the post, unbolt, and disconnect the post from the structure. Replace in reverse order.
Using a single 6x6 post set in concrete makes that more difficult, but thinking about it more it's the ease of getting the replacement in place where I can see the advantage of the other approach. So maybe a PT post set in concrete is just fine for the first pass, and if one ever needs to replace a post you can go back with the two-part solution. Food for thought...
As far as the overall price, the items I desire which I can see adding cost:
* interior posts set in the ground for building out stalls. Maybe it's not needed, but I'd feel better with rafters vs trusses as the interior posts would be tied into the structure (my understanding is with trusses tieing in the interior posts may impact the engineering). If trusses, being 12ft on center for headroom and ventilation.
* more windows for natural light if possible. Having it in the (south facing) sliding doors would be great. Otherwise having it above the doors on the gable ends might be nice.
* cupola for aesthetitcs, but having a fan it it to make it functional would be nice for better airflow.
I plan to finish out the interior of the barn myself - stall walls, kick boards, electrical, etc. Just need a good shell in place to start