You should consider your goals, short and long term, prior to acquiring a frame. Are you planning on racing? And if so is it stage racing, crit racing etc.. Or perhaps you are looking at building from 1 or 2 hour rides to centuries etc.. You really want to make sure that you know what you are planning on doing (again, short and long term) so that you can choose a suitable geometry and frame material for your activities.
You should know the exact dimensions you are going to need to get the perfect fit. Road cycling is all about having a great fit and it doesn't matter if you bike is the lightest or most expensive ... if it doesn't fit then it isn't good for you. This can trickle on down to the components as well (set back seatpost, stem length and so on).
So, in my mind, those are the two most important things to consider and typically I would go about it by planning out my goals and then choosing the geometry to meet those goals. Once you have the geometry (head tube angle, seat tube angle, wheelbase etc...) chosen based on goals you can fill in the rest based on getting the fit dialed.
Really, from there, it is just a matter of how much you do or don't want to spend. Haha.