Build it up from the frame. It will cost more, but the experience of knowing how to do it is worth the extra cost. If you do alot of searching and have the time and patience you can find bargains to offset some of the cost, but it will almost always cost more to spec it yourself. You get the added benefit of having the parts you want, and off the shelf bikes almost always have a few cheezy parts, even if they are mostly top end.