Don’t bother with steel, there’s a huge premium for anythign steel with horisontal/foreward faceing dropouts as the fixie crowd buy them up. I picked one up in 2004 for £20, an identical bike went on ebay a month or so back for £210!
Set a budget (I’d recomend £300-£400).
Stick to the big brands (better re-sale value/easier to sell if you don’t like it and less need to research as theyre generaly all pretty good).
£300-£400 should get you a 3 year old entry level raceing bike (~£1000 new)like a cannondale or a trek. Cannondale are generaly thought to have the best frames at that level, specialized and trek generaly have a better spec (and consequently cannondale are about 1lb heavier and significanlty cheeper as Lance doesn’t ride one).
Don’t wory about flat bars, drops are more comfortable once you figure them out.
Don’t wory about commuters/race bikes, mines been fine for years of abuse. If you want more comfort or arent very flexible, stick a spacer under the stem.