Quite a few of us started out by repairing our old Leak Stereo 20s and
the like. It's still a great road into amp building. for a first step
I'd do exactly that - buy a Leak Stereo 20 and upgrade it with modern
componants. It's all point to point wiring and totally accessible.
Plus you have the leak group as backing to talk you through it. Morgan
Jones even has a suggested circuit for it in his Valve Amplifiers,
which you absolutely must buy. (that's an order!!!)

I think to date I've been through about 30 different circuits in my
Leak Stereo 20!!!