Interesting cool project you have there.
You could look at ACF50 for treating and haulting the rust as its a pretty benign method before looking at pickling and so on.
Looking forward to your updates.
Your quote of £150 is probably very reasonable as it does take a while. The rotary polishing pads and polishing (soap) bars work very well. Its a good thing to do yourself if you've got the time for it. Satisfying job.
Very nice build. Something very, 'right' about it, looks great.
How do you find it to ride? Any thoughts on the alloy forks? I have the carbon versions which I really like, but I did wonder how the alloy ones would be.