IthaDan's suggestion is a great one: buy a complete wheel - hell, get a 27" wheel if that's what your frame is made for - and then completely disassemble it. This way you absolutely know that all the parts will be the correct sizes, etc. because it already came put together. So there is no guesswork for you on the part list. Then, when you rebuild the wheel you will have lots to learn about lacing spokes, trueing a wheel, etc. You absolutely will learn about wheel building this way.