NO DON'T DO THAT for that way, my friend, lies madness.
Seriously, though, it really depends on what things you'd like to add to, remove from, or change about the game. What floats your boat? (I mean aside from yelling "ANBLASEN" and blowing air into the tanks.) You may only find that out by playing a while.
Do you want to change or enhance certain elements of gameplay?
Add a bunch of great-looking ships?
Get rid of some annoying little thing that always make you cringe or grumble under your breath or shake your fists at the sky?
Enhance the graphics?
It's pretty much all out there, or at least as much of it as folks have been able to figure out how to do so far (and new stuff is coming along all the time).
My first recommendation, though, would be to take a look at SH3 Commander, which is not really a mod but a stand-alone program that will give you a variety of new and/or easier-to-access options for managing your careers and changing certain aspects of your gameplay.
Full details here:
http://www.subsim.com/radioroom/showthread.php?t=147237
And you can download it from the Subsim Downloads. Do a search for SH3 Commander and it should pop right up. If you are playing GWX you will also need the SH3 Commander + GWX3 config files, which should also come up in your search results.
