Get Medieval II and install the Stainless Steel mod.
Never buy Empire: Total War. There are no more major patches planned to be released and the game is still horrendously broken.
Well he Imperial Splendour Mod in my experience has made it quite fun.
The Mughals, Denmark, Portugal and Persia are playable; the factions are unique and more realistic and the economic situation is noticeable different.
I have both SS and IS and like both; however, as far as unique factions go, as far as naming and diversity I find IS a bit better. So many units, diff. cannons, cavalry, and all the such makes it quite fun, heck even unit sizes are tweaked realistically, for example:
Russian Inf. Unit: 180
Swedish Inf. Unit: 80
Prussian Inf. Unit: 135
The Russians have large units but bad morale, not so good accuracy, and all right hand-to-hand.
The Swedes have great morale, good accuracy and maybe the best melee stats.
The Prussians are tough, good shooters, durable in hand-to-hand and good accuracy.
I like the mod myself.