www.map-review.com) will help you with modding...
http://pcgamemods.com/5253) is an NPC tool, it helps you make NPCs.
I'll answere any questions you want too.. I can map script and skin.. or at least I try my best, lol.
EDIT: Oh and.. uhh... about where the menus are.
Check in your Assets1.pk3 file, under the UI directory every menu in the game is listed there. If you don't already know a pk3 is simular to a zip file, and can be loaded with a decompression program, like winzip or PakScape.
Every game file, from the floor texture, to the background music are stored in those PK3 files. It saves space, and keeps people from tampering them. BTW, do NOT ever change those PK3 files, make a seperate pk3 file if you are going to experiment with modding... any pk3 after those will be read also. If you do edit any of those assets pk3s your game could malfunction crash, or just not work at all. You could probably reinstall if this happens though, although I wouldn't recommend it.