My advice is just buy a new rear wheel. If you've never built a wheel before, and you set out to make the perfect one as a way of saving money, you will probably be disappointed. If, however, you set out to just learn how to build a wheel, you will pretty much definitely be successful, although you'll probably have a weird rim and a lot of extra money thrown in for mistakes.

My LBS built me a CR18 on a whatever formula hub for $130 (CDN). I take it up and down curbs all the time, and the spokes are still tight. AND your LBS will usually give you free truing/tightening if they build you the wheel. At least they damn well should. And yeah, rock the 27s. For one thing, not many people do it, and as you say, it'll keep the BB where it should be.