hey dan, your new best friend will be this site
www.guildwiki.org but as a answer to your questions
1. Is there single player mode?
No, how ever the game can be played with no other human interaction (using the henchmen and NPC traders to buy stuff)
2. Can you do quests with friends, otehr people?
Yes, you can do any quest, any time with any friend, however you'll only get the reward once(with an exception for those small amount of repeatable ones) and if you have beaten it before you have to make sure you friend has the quest in his quest list ( L key if i'm not mistaken)
3. Do you think i did the wrong choice?
No, i really don't, factions and propehcies are both essentially a good game to start with, however factions probably is a better choice for you, because it gets into the game faster, you don't have to worry about being halfway through the game and only level 17 because in factions you'll get a good amount of exp for stuff you'll do,
hope that helped >.<