The developers have continued to provide free patches thru the entire life of the game (and another in the works). So unlike other games, there arent any must-have player created fixes IMHO. What is a very VERY large factor is that you download the latest patch (only the latest is needed to catch you up) and install that.
http://www.shrapnelgames.com/Illwint...DOM3_page.html
CBM is the only one that might be considered IMPORTANT to install and use. And maybe EndGame Diversity.
But even thats a split opinion. CBM is supposed to balance the game but it makes so many changes that it becomes a choice of either knowing how to play with it, or without it. EGD is considered to be a fix for the limited end-game strategies that seem to show up with the cbm crowd.
My personal opinion is, why shorten the game? Dont be in a hurry. You can enjoy a year of solo play learning each of the nations and its quirks. If you find its imbalanced then take a look at one of the (3 at the moment) balance mod projects. If you decide you like solo and want a better AI then use one of the AI improvement projects. Or just the mods that add more nations into the game. More maps, more nations, more spells, more equipment.
And of course, once you find a nation that totally fits your gaming style, and some strategies you feel work well for you.. then there is always multiplayer games. Stretch it out. Enjoy it.