FYI, if building from zero (ie: no parts in your parts bin), piecing together a bike is rarely less expensive than buying a complete bike. IMO, building from the ground up makes sense only under the following scenarios.
a.) you have a significant parts stash and only need to buy a couple items to complete the build
b.) you or a friend have enough mechanical savvy to not completely screw it up, and have to go running to the lbs for expensive repairs of your mistakes.
c.) you really want to learn how to wrench.
d.) you plan on quickly or immediately upgrading enough parts from a stock build that it makes sense to just spec and build yourself.
e.) you have access to ridiculous deals on parts that bring the total cost in line with a complete bike.
Both the cheapest and middle of the road Motobecanes are pretty good deals, if you or a friend can make the necessary checks and adjustments out of the box. However, don't feel obligated to buy now on eBay... these bikes are always available on the bikesdirect website. Also, bd is planning on releasing several 'cross bikes soon. One of these may fit your needs better, so maybe a little patience and research.
That said, building your own bike is a great way to learn about it, and I highly encourage everyone to try it, as long as they are aware that the costs may increase relative to a complete bike.