Nice, but there's plenty of good looking 531 steel frames out there. I've always thought that buying a used frame as the basis of a build was a guaranteed way of making it far more expensive than you expect. I'd rather have a complete project and maybe replace some bits rather than start with nothing and have to source everything needed piecemeal.
Yes, starting from a frame can be significantly more expensive, but if you're after something specific you often have to just bite the bullet and pay for it. As an example, a couple of years ago I decided I wanted a classic 531ST tourer - Raleigh Randonneur/Royal, Dawes Galaxy, FW
Evans... something of that class. I also wanted some specifics in components. I found a Raleigh Royal frame locally after only a couple of months, and I bought it and built it - and I love it now, but it cost around £250 (including new wheels). Had I held off and waited for a full bike of suitable style to become available somewhere where I could get to it... I'd still be waiting (and yes, I'm still envious of whoever got that Randonneur for £46
).