they won't work on an aluminum 120 spaced rear frame, you'll need to find a 130mm rear spacing frame or set the fork ends of a steel frame farther apart to use those wheels.

if you want something close to 18 lbs you may be better off buying a complete bike. reynolds steel is usually better but it really depends. i haven't heard of many bikes using 725. 520 is basically 4130. my bike is 520 and i like it a lot.

i would suggest the IRO or a slightly more expensive frameset like a gorilla or a traitor bike.