Sir,

Well, the first thing to take in account is the geometry you want. Once you determine that look for frames that offer you this.

To get under 20lbs. for a complete bike will be a bit hard with 105 and a steel frame but maybe not impossible.

Also another thing to decide is if you want to buy a frameset and the parts separate. This will give you more control of the bike as many complete bikes give you cheaper parts to keep cost low.

I am writting from Bulltek Sports - www.bullteksports.com and if you want you can contact me (Xavier) for more details and advice.