Cheapest option if you want to do it to say you did:

1) Buy a complete [machine built] wheel.
2) Take it down to bare hub, a bar rim and a pile of spokes and nippes.
3) Buy new rim tape.
4) Rebuild.
5) You built a wheel!

e: echoing the statements about frame quality. That might be designed aroun 27" wheels. Heck, it might even have 27" wheels. It's not a huge thing, but if you're like me, it'll get your OCD to flare up if you build a 700c wheel for the back while the front was a 27" wheel.