The issue with building a wheel is not switching the cassette, it is lacing the spokes. You can teach yourself, but it isn't the easiest skill to acquire.

As for brands, most of the big brands making road bikes are similarly priced, to tell the truth. That is, you'll get a similar quality frame and components for similar money. If you are buying new, and you want a light bike at a reasonable price, you'll be looking at aluminium frames. A giant defy 5 is a good entry-level road bike but it is about $700. American members here will be able to help you more, or it might be worth visiting a LBS and talking to them about what you might get for your money.