I tend to use the Combat Mission Mod Database,
http://www.cmmods.com/ for all of mine, which provides mods for all 3 versions.
As for terrain, well that's so much down to personal taste I would hesitate to recomend any, and it also depends on whether you want 'gridded' or 'non-gridded' terrain.
The database comes with a search facility so you can search for say, 'terrain' or 'grass', (which takes a lot of the leg work out of it), and most of the mods come with previews, so I would simply suggest just searching a few, and see what takes you fancy.
Happy modding