Building a bike yourself would be much more expensive. If you are very particular about your components, then it makes sense to do it. If there are only a couple of things you want to change, then it would be better to buy the complete bike and the components you want. If you LBS is willing, they might work some trade or swap for you. If not, you could just sell the components on eBay.

If you want the experience of building the bike, just take it apart when you get home and reassemble. Or, some shops might sell you the bike still in the box. Most manufacturers demand that the bike be built, but we've done it for a couple of customers. The LHT's steerer tube comes uncut, I would recommend you have the LBS cut it to size if you want to assemble the bike yourself.

I'm pretty particular about my components. Even then, a complete bike would make more sense to me. There is not much on the LHT that I would swap out. I would swap out the wheels, the saddle and the brake pads. Everything else on it would be ok for me. There are some things on it that would not be my first choice, but they are good enough. Pretty much the same is true for the Trek.