I think you did just fine given the time you've been in the hobby. The only criticism I could add would be maybe next time mask off the headlight lenses and you'll get a cleaner look to the black you painted. But seriously I think you did good.
MDF is actually quite affordable and if I remember correctly you can buy them in 2' x 4' pieces at HD or Lowes. 4' x 8' sheets are more cost effective though
Back in the 70's I built the Revell Cutty Sark model when I was with my first wife.She was baby sitting some child who thought it would be fun to pull off the rigging and break 2 of the masts. My wife at the time didn't think it was a big deal. I had some serious hours into the rigging of that ship. That and a host of other reasons is why their is a second wife now.
I know nothing of dioramas, but I do know wood. Wood tends to bow and twist over time. To help prevent this make sure it is dry and then sealed on all 4 edges as well as the top and bottom. I would guess a more stable product like dense foam board would work better than wood.