It would be much more economical to buy the complete and swap the bars later if you choose to do so. The cost of building an LHT with the same components as the complete would end up being considerably more than the actual complete. Personally I'm a drop bar fan but it would be easy to put a flat bar on the LHT. As far as sizing goes you'll probably get all kinds of recommendations on what other people THINK you should be riding but the only way to really know what will work for you is to get out and test ride some different sizes - either at your lbs or through your local cycling community.