I first got into this style of riding by converting my old Vassago Jabber into a mtn bike with drops. It worked great for about a year or so but then I just started to feel a bit stretched out. Well, that's an understatement. I was waaaayyyyy stretched out. It became uncomfortable so I decided to pick up a frame/fork and throw all my parts on it. It's the main bike I ride now. It's a Soma Wolverine with carbon fork.
I say this because the drops will push you out much further than the flat bar. Plus, you'll need to get long pull lever brakes or brifters, depending on how you want to ride it and assuming you go disk brakes. If you only need the brake levers, Tektro makes long pull brakes that are dirt cheap ($25). I've been using them for almost 2 years now and no complaints. As for other components, buy what you want, are comfortable with or can afford. I'm running mtn bike cranks, gears, and derailleurs as that's what I had on hand. I plan on upgrading the cranks at some point. Hopefully others will chime in that can explain things much better than I can.
*Custom Samsara steel SS *Soma Wolverine gravel-er *2004 Lemond Tete de Course, Ti and Carbon baby! *Vintage-ish (2007) Vassago Jabberwocky - Back to life!