Alfine 8 speed is Alfine 8 speed. any of the three you've listed will work with your hub. Finish and features vary somewhat. the 503 allows multiple gears to be shifted in one lever movement, depending on how far you move the lever, AKA: Rapid Fire... (personally, i find them annoying)... you may or may not find that to be an advantage. Point is.. This is YOUR decision. Make it... and i've never seen a "Left side Alfine shifter", ever... which is not to say someone has never seen one, or that one was never produced..... DECIDE For Yourself.
also... Alfine hubs come set up for either Chain drive or Belt drive.. most are chain drive... Chains are more efficient, and more versatile. you can't cut a belt to fit any bike, and finding one that is "just right" can be tricky and time consuming in rural America, or even some large cities. Buy a Spare belt.
also.. verify your Handlebar O.D. .. MTB Bars are about 22.2mm, Road Bars are about 24mm.... i'd imagine these shifters come in either diameter, clamp-wise...
