If I were going for low trail on a budget I'd just get the
DIY Velo Routier and be done with it. They've already picked all the components with pretty good bang for the buck. As others have mentioned, you're starting with a pretty heavy frame to begin with so there's no reason to go nuts on the parts, and as you have already figured out, it's pretty easy to get into the big-money range when you start adding up a bunch of nice components that you're hanging on a pretty cheap frame.
Aside from the aesthetics (which is reason enough) I would never consider putting sora on a GR frame because I can't stand riding a compact crank on long rides, but that's probably just me. Nobody else seems to mind very much.
I would also recommend against going with dynamo lighting too soon now that battery lights are so good. You can always get that later if you find yourself doing a lot of 400's and longer.
10 speed vs. 9 speed depends mostly on what shifters you want. DT or bar-end I don't think it makes much difference as long as it works with your shifters. With brifters the group will dictate the number of gears.