Read this -
https://www.m3post.com/forums/showthread.php?t=1418410
Engines are essentially the same regardless of year, assuming running most up to date software. If its a manual, 08 had some flywheel / clutch differences I believe.
No one knows what will happen to values. However its safe to assume E93's will be the last to go up in value (if at all) as they are relatively unloved by enthusiasts.
Cost to run it depends on how much you drive it, and whether you buy one with most stuff (brakes, tires, fluids etc.) relatively fresh. When stuff goes wrong it can be expensive, its German.
They are great drivers cars, excellent steering, plenty of feedback, sound great and are exceptionally well balanced. Very comfortable for long trips, for highway cruising its basically like any other 3 series with stiffer suspension. EDC in comfort rides very well, and depending on what exhaust setup you have it's pretty quiet. The car has a significant amount of sound deadening.
Buy the owner, condition, and history well and you cant go far wrong. Good luck!