In reply to MikeR:
I've raced a few triathlons with a clip-on tri bars on a normal road bike. Alu bars are fine - if you go carbon, then check they are designed to take them. Depending on the design, you may want to put a shorter stem on for racing and/or you can also shift the saddle forward a bit to get a better position.
I'd suggest investing £100 in a proper bike fit, tell them what you want to do and take their advice on geometry as a starting point. When I bought my last bike, I explained I wanted a race bike I could ride all day and built the size/geometry/position around that.
Once you've got the frame size and geometry, then you can start shopping around/looking on eBay and comparing specs - bars, stem, chainset (crank length), etc, etc, can all be changed to for what you want, as long as you have the right frame.