Darn shame that there’s nothing quite like Heathkit around (even though there’s a company by that name making ham radio stuff). I learned a lot from them, and modified almost every one I built!
But I’d poke around the Internet looking for electronic kit manifacturers. I’ve found that, in addition to step-by-step building instructions, they tend to have explicit circuit explanations, too.
Have fun and learn like crazy! Don’t worry too much about mistakes. They are the best teachers.