I used a 4130 Cromoly tubeset from Nova cycles in the US before finding out that Ceeways who do tubesets in the UK is only 20 miles from home. The dropouts came from Paragon Machine Works and were stupidly expensive. I designed the frame using RattleCAD which is free on t’internet (geometry in pic below). All the tubes were mitered by hand using a file and dremel. The frame was tacked together using a jig made out of 20/20 extruded aluminium then fully TIG welded in my cheapy Lidl bike stand.
As to background reading there is a thread on a mig welding forum (think the author is on here too) that is pretty good for inspiration and the Velocipedesalon forum is well worth joining if you fancy having a go. I have done a write up on Sketchymtb and will post the link if I can find it.
I have to say the first few rides are pretty scary waiting for it to fall apart but after that it is fairly satisfying knowing that you’re riding something you have built.