You have very messy handwriting...
Joking, joking. =P

OK, where to begin. First off, are you sure you know what you're doing, or more precisely, do you know what you're getting yourself into? Saying that you're going to make a mod isn't something you can say lightly (unless it's a small, little mod) - make sure you'll actually work on it, and if you note that in the first few months that you just can't work on it, say so.

Next, do you know how to modify large portions of the game at the moment, or at least, are you willing to learn how to modify the game yourself? Even if you've ten, twenty, thirty people working on your mod, the mod leader will always need to be there working on it even when others have come on gone. Remember, although you might be able to a mod a text file here and there, unless you've got a really big team working on it, you'll need to be the "Jack of all trades" sort of guy (Not only add a unit here and there but add buildings, add resources, edit the campaign, add factions etc.)

If you've answered yes to the above two questions, might I suggest trying out other mods that aim to add to the game like yours. Have you tried the XL mod? BKB's Super Mod? Wes' MedMod? They all aim for something along the lines of your mod - why you might ask? Well, if you're ever stuck on idea's "borrowing" them from a similar mod always helps.

Also, try looking through the Repository for guides (or guidelines) on modding, and if you can't find what you're looking for, don't hesitate to ask in the Alchemist's Lab. One last thing I might add, always keep in touch with the people looking forward to the release of your mod, always provide updates on your progress as often as possible or else they'll just forget about the mod and it'll disappear along with many mods before yours. Or you, yourself might disappear like many would-be modders before you.

Oh, and if you ever need someone to write up descriptions for factions or units, pick me. I'd like to help out more so, but I lack regular internet access and it'd be difficult for me to stay in touch.
Otherwise - Good Luck with your mod (And give it a name too, be imaginative here, don't just call it "highlandclansman's mod" because that would just be boring )
And remember, Raz = The guy that writes good descriptions.