The rocksmith game might help you.

Even guitarhero/rockband will help you with using your fingers and will help when you want to try a real guitar. Muscle memory might not be great but for someone who is just doing it for fun it will be helpful getting your fretting and strumming coordination. And them being games might help fend off the adhd enough to keep you motivated to pick it up again after putting it down

I started with piano, technically, but I was 3-4 years old, and don't remember any of it. I can sit a piano and make it sound good, but I can't play sheet music on it. I switched to violin in second grade, and then just learned how to play everything except for rhythm guitar, and piano. Chords mess with my fingers. Strangely enough my ADHD allowed me to super focus, but I never got the hang of sight reading, so I mostly play by ear, but no one can tell.

I can even play a didgeridoo, and that required me to learn circular breathing.