Ninjon, Miniac, Duncan Rhodes, Squidmar Miniatures, and Goobertown Hobbies on YouTube (or whichever frontend thereof) have some awesome tutorials. Be forewarned, you will get sucked in!
Not really, I just kind of wing it, honestly. I'd recommend printing in white or gray to begin with, but after that I just go for it. I figure if I completely screw it all up, I can just print another and start again, lol. I'm not an experienced painter by any means, but I enjoy it when I do. It's very relaxing to just throw on a podcast and zone out for a while.
I've spent quite a bit of time finishing and painting models, but have never been overly satisfied with the finish. I've used wood filler, spackle etc and assorted paints, but if you have some tips they would be appreciated.
You're doing way more than I have. I've done some light sanding and smoothing, but I mostly just paint raw. Yours looks great. Can't even tell it was printed.