I have one of the entry-level Dawes road bikes that I got for under $200 a few years ago (now you can get them new for around $300 shipped from bikesdirect.com or from some ebay sellers. Mine has a 59 cm frame and weighs around 29 lbs, it has 7 gears on the back and 2 on the front (not super-easy gears for climbing), it has "retro" stem shifters. Many cyclists want a lighter bike with brifters. I've tried (and own) lighter bikes with brifters and still find the Dawes Lighting Sport very adequate for my riding (I just rode 12 miles this evening on it). So my opinion is that you can get decently started for $300-$400 (you could even get a helmet and bike shorts in that budget).
You might be able to find a big-name used bike (that was like $1000 5 or 10 years ago) for that kind of money (I don't find those bikes for sale where I live).
You seem like a very serious-minded person who always wants to make the perfect decision. I say just find a road bike to ride and (no matter what bike you start with) you will likely (if you get into this biking activity a lot) want another bike. Why? Because it just takes time to really figure out what you like and what you want to "try-out" in the future.