i have a Masters and a PhD in electronic engineering ;) but i'm not using any of the stuff i learnt for these mods thats for sure.
theres alot of very informative sites about modding tube screamers and boss sd-1. i suggest this would be a good place to start with modding. other than that i just google around and see what other people have done, and combine with my own ideas.
my lead 12 was 45 pounds plus 15 delivery. reverb 12s are about the same price, or a bit cheaper. they are built like tanks, sound quite good already without mods, and are easy to work on. it might be a bit expensive for you, but i would still suggest it as a good place to start. with a few simple tweaks, like an external output and a new opamp you can hear the difference instantly.
i wish i could suggest the epiphone valve junior, but i'm really not keen on that amp :( i'd quite like to put my lead 12 up against a modded valve junior.
my next move for this amp would certinally be to play around with the tone stack. but the vintage 10 was a bit of a dissapointment, so i've lost a lot of motivation. the lead 12 tone stack works a bit less powerfully than the one on my JMP50.
i'd really like the oppertunity to design a practise amp one day. i have alot of ideas to design a quite nice sounding low cost amp. if they could do it in the 80s, we should be able to do /better/ today - not worse.