A couple questions

[Con]

Could I make a script that makes the spartan go prone, or a health dispenser which can heal both yourself and your teammates? I want something other than simply spawning a health pack.

I don't know much about halo script, but I do know that scripts are iffy. Even if you could make a script that made you go prone, you'd likely get a lot of syncing problems. You would be better off replacing crouch with prone animations rather than tacking on features. I'm pretty sure the health thing is possible, it just has a damage effect with negative damage (gives you health). Wait for someone like CAD to respond.

How could I make a model exporter and importer for Blender? I'm thinking The Ghost could help, seeing as he made that third party exporter for 3ds max.

It's a lot of work making those, and we don't even have one for Maya yet. If you can model in Blender and move to Max for the rest, do it.

Do I have to use HaloScript to script in HaloCE?

Yes.

I have no interest in getting 3ds max, I've used Gmax and it was terrible.

Like it or not, your map's going to have to go through it until a plugin is made for Blender. I don't see that happening any time soon considering hardly anyone uses Blender.