Welcome back to The Great Unwashed of FS2004.
To me, the obvious first changes are to make sure you install the FS9.1 update and the "No CD" patch.
The next thing I would install is FSUIPC.
After that, you might want to concentrate on your "skybox", sun, clouds, and water textures. There were also some handy updates for things like stars and the moon. These were downloads that used to be listed at flightsim.com as "must haves" for FS9. After that, I would be looking at updated mesh and ground textures in the parts of the world you want to be more detailed.
After that, it really depends on what Era you're shooting for.
1900's, 1930's, WWI, WWII, 1950's, etc.
I switched over to FS9 from FS2002 back in 2015-ish and I'm
still building things out.