Remap Caps Lock
Applications -> Keyboard Layout -> Options...
Caps Lock key behavior -> Make Caps Lock an additional ESC
If you want it to be another modifier you could make it an additional Hyper for instance.
As far as assigning shortcuts, nothing beats Compiz's Command plugin. Sadly, Unity seems to have crippled it. You can try your luck though.
Applications -> Software & Updates -> [x] Community-maintained free and open-source software (universe)
Open a terminal
sudo apt-get update
sudo apt-get install compiz-plugins
Applications -> CompizConfig Settings Manager -> Commands -> [x] Enable Commands
Set a hotkey:
Commands -> Key Bindings -> Run command 0 -> Disabled -> [x] Enabled -> Grab key combination
Set an action:
Commands -> Key Bindings -> Command line 0 -> <input command here>