You seem suited to take on the project, you already did the others. French bikes aren't the easiest, I guess you hadn't enountered some problems they're know for.
I didn't read the link, read most of what you wrote, well read it all but will only address some. Fond of vintage my self, I totally agree with the correct this, the proper vintage that, too much. Too much money and trouble in many (most) cases.
U08's like others are very different than Italian, English and Japanese bikes. A Japan built bike might not be the most rewarding, it'll be the simplest and perhaps the more durable choice, they're also under-priced. That too is just beginning to change.
400-700 will buy you alot of used frame. Building it up can double that amount even if used parts are chosen. A new frame other than a cheap Al. one or just just a few chromoly steel ones can cost 400 or more, not many are even that low.
Any frame other than yours now will be WAY different; "good, bad not indifferent"
Funny that complete bikes actually "cost less" all things considered. To buy a used frame for 100 even then to gather used parts & wheels, W/shiping 200 more; it's hardly worth the effort and I'm being conservative here.
A complete bike is better, you can det. it's straight, rideable and if it has some parts you like, keep them. All that's not to your liking, proceed from there.
The only compelling reason to start from scratch is if you can do it no other way. Choose a bike or frame with 700 wheels, most will have chromoly tubing. Make sure that it's equipt with the shifters you like. Bikes with brifters(br.lev.shfts) start at about 250 used, depending on where you live or shipping added (70-100). Others can be found for 150 or so. 27" wheeled bikes can cost 50. Many 27" wheel bikes won't give you a rewarding ride and statistically most are HI-Tens. Steel.
Your urge to "do" a bike needn't include from scratch, I've made that "mistake" pleny of times. New is another story. Check out the Surly Pacer or Long Haul Trucker.
That's a start.