Me, I'd build a bike on an MTB frame. Get one close to what you want in size from Craigslist and go from there. You can dump a cheap (useless) front fork and get a nice CroMo fork for less than a C note. A lot of the frames came with lugs for disk brakes, even if fitted with V brakes (which are lighter). Most will accommodate much bigger tires than most road frames. Most are as tough as nails. I'd go aluminum frame and work on the weight of the components. Put drop bars on if you wish. It'll be a "cross ~ gravel" bike. But it'll handle any road conditions with ease