1) Assasination is better for leveling.
Nice burst plus Multilate can be used infront of the target. If you go with sub, you can not use backstab infront of the target.
Combat can also work.
When you ding lvl 100, switch from Assasination to Sub for dungeons... alot more dps

2) Draenor. Do quests and dont do any dungeons before lvl 100. The exp in dungeons is terrible.
Do all the Zone missions that are in a new area, they give a ton of EXP... a big boost
3) Mark of the shattered hand until you have ilvl 660 and above. Then go multistrike proc for Sub and Haste proc for Combat.
4) I prefer Enchanting since you can disenchant items you find you can not use. The gold you make from the enchanting mats are usually alot more than the gold you would get from vendoring.
+ Enchanting is a huge gold maker on AH.
the secound profession is up to you... Alchemy is good for gold making too.. everybody needs flasks in raids and you have a free Herb place in your Garrison... = free mats for the gold making.
Here's my UI that might help
http://i.imgur.com/4KKslal.jpg
thanks all but this detailed response wins by far. appreciate all the help fellas. 250 fg sent! thanks bro!