Curse of Exhaustion is meh in my opinion. Coupled with Amplify Curse it's ok but Warriors can Intercept + Hamstring, Rogues can Sprint + Crippling Poison, Druids can remove the curse, etc. so PvP-wise it's not terribly dependable. It may have some more use in PvE, especially with a drain tanking build.
Both Imp. Drain Life and Imp. Life Tap are good choices if you're going deep into Affliction. There's really no other talents at that level that's worth taking anyway (Imp. Curse of Weakness and Imp. Drain Soul are crap).
As for the increased range talents, they're both good in my opinion but Grim Reach may be more of a priority if you're the DoT/Drain Life type.
Grinding pet is competely up to you. Something to keep in mind is the Succubus can sustain DPS, even if she's out-of-mana, whereas the Imp blows its load and then sits there verbally abusing you. The Felhunter wouldn't be my choice for grinding because it doesn't do much damage and it certainly can't hold aggro (Paranoia spam, oh how I miss you).