I can comment on that. Search for and download the HEK (Halo Editing Kit) and on the HaloMods Files forum and other forums such as the Gearbox Software one, you should find lots of tutorials to help you accomplish making your first map.
3ds max too is a must have for this. You can download the trial at discreet.com.
Also, banshee hanger sounds nice. blow holes in enviroment is entirely possible, just takes work, because you have to build it in pieces and layers. That's what i'm doing