No need to buy them in order.
The two other campaigns (Factions and Nightfall) are stand-alone games much like Prophecies, and they have their own storyline.
The expansion (Eye of the North) ties up several loose ends from the Prophecies storlyline. If you're looking for a continuation to Prophecies, this is its spiritual successor.
In order of purchasing, I would recommend Eye of the North -> Nightfall -> Factions. That's in order of age, Eye being the newest and thus most populated. To be fair, new areas/items opened up in Prophecies and Factions have revitalized the population in these areas.
Look around and you can find Eye of the North selling for low prices too.
As for having someone to play with, you can join a guild so they'll help you get back up to speed
