New content is always welcome, but I would suggest starting off by concentrating on something smaller, and then adding to it, rather than trying to do something huge all at once. I've been working on a pretty big mod (Sun and Moon) for quite a while and I've been tempted to give up, but once I decided to focus on just two factions instead of four, it became a lot less daunting.
I'm pretty sure that new load screens are possible, but there isn't really a "story mode" in Glest yet. Currently the closest thing is just to make a bunch of scenarios and number them. That way, the player will know what order to play them in order, but there's nothing stopping them from playing them out of sequence.
Regarding 3DS vs. Blender, Blender is definitely the way to go unless you already have a copy of 3DS and know how to use it. There's no point in spending hundreds (or even thousands) of dollars on it when you can get Blender for free. Also, almost everyone on the forum uses Blender, so there are fewer problems with sharing/collaborating if you use Blender.
Are you planning on doing this whole mod yourself? If so, you'll need to know modeling, texturing, animating, XML, and Lua. Fortunately, you already know how to use the GIMP, so texturing should be no problem, and XML is very easy to learn (at least enough for Glest-related purposes). I really have no idea how much of your skill from GMAX will carry over to Blender, but just the fact that you have modeling experience should help a lot.
Since you're starting off with Orcs, is this going to be the usual Tolkien-style high fantasy sort of thing?
Anyway, I look forward to seeing what you can create.
