You can build on regular lugs and have a sloped top tube. The only required adjustment would be in the chainstays and the head tube angle. It would depend on what head tube angle you could tolerate. The CS adjustment has a lot of potential fixes.
Or as you say you could braze it.
If you seriously want to learn custom frame building then my advice would be to learn never to make compromises. The absolute requirement to success is to fit yourself, and the best geometry. For all the time and money you will still put into this thing, a buck or two for the right fit will repay you many fold.