I like your idea of building something that suits your needs instead of some of the hodge podge parts that come on the Brompton. I own a Brompton, and with the knowledge I have now I wouldn't buy the same bike again. I would still buy the Brompton just configured differently. So I would buy a basic Brompton in the colour you wanted and then sell off the parts you don't need. A basic single speed with no fenders or racks etc. This way it would be cheaper than trying to acquire the basic frame and fork.

Again if I had the choice to do it again, I would be an extralight version second hand because they are almost impossible, at least in Canada to purchase that version. Then sell off all the parts I didn't need.

Good luck with your project, it sounds fun.