to give a small list of pro's and con's :
Prophecies :
+ nice learning curve, seperated tutorialish area
+ good story line
+ Probably the cheapest (be sure to check out the
Platinum Edition, which basically is GW:P and GW:EN in one)
- the henchies suck, and so do most pugs, so you might be screwed till you get to know people
Factions :
+ Nice, relaxed PvP options
+ Elite missions
- very steep learning curve
- short storyline
Nightfall :
+ Heroes
+ nice storyline
- Newest campaign, so still expensive ...
Ranger goes with pretty much any secondary. However, for the sake of being able to resurrect someone more than once, monk or ritualist would be nice. Later on you can switch your secondary profession anyway.
if you want to play PvE with your friend, you need at least one of the campaigns he owns aswell (so you both have access to the same content. Other than that GW is "one big server" (ok, not realy, but you can meet up with everyone easely)