Any generic aluminium frame with a generic carbon fork. Assemble, ride, enjoy.
Or, if you are happy to go second hand, the older Cannondales are well thought of. The ride is fairly frim though so 700/25 tyres are worth thinking about.
Here are a few of my recent budget builds:
a €66 Jan Ullrich frame in my size, set up here as a tri trainer bike. There is about €200 in this bike. It now has a full aerobar cockpit and rides very nicely indeed:
http://i271.photobucket.com/albums/jj158/simon_milan/March2009147.jpg
a €50 Fort aero road frame and a €50 Wolf fork. All in about €375 for this, but the wheelset (Felt TTR2) was €150 and I wanted the Halo seatpost from Blue:
http://i271.photobucket.com/albums/jj158/simon_milan/IMG_3376.jpg
I have just finished building up this Cannondale ´93 R800 into a flatbar commuter. Frame, fork, brakes, crankset and BB, pedals, headset, FD and RD (all 105/Ultegra 600) was €74. Crappy picture from the original eBay offer, sorry:
http://i271.photobucket.com/albums/jj158/simon_milan/cannondalepicture.jpg
My steel roadie is a Pug Comp 5oo with a generic carbon fork, running 9 speed now. Lovely ride, all in about €300 I think:
http://i271.photobucket.com/albums/jj158/simon_milan/008.jpg
Best of luck with the build. Lightly used second hand frame/fork in your size should be easy to come by and cheap.