Hi tomdy2k,
I would suggest that you learn SE4 with the stock Version. Most mods will add new components and technology that will probably make it harder to learn. You can try TDM mod, though. It doesn't change the data files and behaves like the normal game, but the AI is better. Apart from the TDM races there are also other modded races that will give you a better challenge than the stock AI races. From the top of my head I recall the Vaxin, Orks, Minbari, Cephalopeia and Space Vikings to be good non-TDM races.
Have fun
,
Rollo