Thanks Andy!
Yes I am planning on building the fork myself, in fact it will be the first step in the project, and if it turns out I am not able to do a proper job I will transfer the project to a local willing frame builder. I hope learning curve will be manageable though and I will do a decent job with the fork. For headset I am thinking tapered, integrated.
And yeah, I guess my only general idea for the down tube was thicker walls. something like 0.9/0.6/0.9 or in the vicinity of it. I do like the idea of an oversized down tube (40-44 mm) but you might have changed my preference, it does make sense that smaller diameter does not dent as easy.
I do not care about aero either, but would not mind learning how to account for "compliance"/ride quality when I choose tubing. It's such a vague term though, although apparently is what differentiate a custom made frame (made by an experienced builder that is) from a factory one.