For your enderman problem and to add morre mobs: install dzhark's mo creatures and dzhark's custom spawner. open options after installing and click global mod options. if you poke around enough you can change spawn rates of certain entities such as endermen.
And a few of my favorite personal mods:
Mekanism
Magical Crops (grow coal, diamonds, iron, YEAH!)
NPCWand
Forestry
Natura
Thaumcraft 3 (Magic to a science, really fun)
COFH Core (When you know how to, you can change spawn rates of minerals down below, you could make diamonds spawn in clumps of 50 if you want!)
Portalgun(Difficult to install, but fun)
Pam's Weee Flowers
rottenfleshtoleather (turn zombie meat into leather by cooking it )
Balkon's Weapon Mod.
Recommendation: My modpack may have be perfect for what you want. It has most of the mods listed and more. I can finish fixing it's download problem. If you need help on how to get my pack, ask. Check this out after a while. If it downloads and doesn't work (or if just plain vanilla loads) then Im still uploading and editing download links.
The link to check out after a while:http://www.technicpack.net/modpack/details/you-may-not-be-alone.397135