Well..... it depends how much money you have.

However, as long as you get a good frame as aero as possible, then you can upgrade everything else.

You can get a bladed seat post, change you aero bars, put bull horns on the bike, get the type of wheels you like...

I have a Giant TCR Aero2. And we have slowly modified it from the shop model to the TT specific bike it has become...