I'd stick with 9-speed. 10-speed stuff tends to be tailored for the narrower chain (and it's likely more expensive).
Shifter and derailleur pull ratios need to match. Fortunately they do with all Shimano 9 and 10 speed road components.
+1
Any 8, 9, or 10-speed long cage (GS) derailleur will work. Stay away from 8-speed Dura-Ace, but there was never an 8-speed D-A long cage RD anyway.