I answer reader email!
The number of games in the OSR is overwhelming. I think the best place to start is with a retro-clone.
The implied setting of Original Dungeons & Dragons was post collapse not post apocalypse.
Beware the pitfalls of putting too much interesting stuff in your adventure scenarios.
When you play the game you get a story that is easier to create and probably better than the one you intended.
Are you going to ride the Story Horse or the Game Horse?
I've developed/borrowed/stolen a cosmology for my Swords and Wizardry games. It has been in my head for quite some time but not written down. Hopefully, it is somewhat coherent.