You might want to look at www.ceriatone.com. They have schematics and sell main component boards for marshall 18W, JTM45, Plexi 50W and Plexi 100W amps, as well as a number of Fender amps, two Matchless amps, and a Vox AC30 minus the top-boost board. You could also check out ax84.com - they have their own line of amps.
Having just finished building my first actual amp, from a kit with a chassis and boards supplied, I don't think I'd have wanted to start from scratch on my first one, although I suspect I'd manage now.