Also, you definitely will need to do PT2 to get to level 50. PT1 pretty much will start getting you 1-10xp per kill in most places, once you are around Level 35, with all sidequests/mainquests completed. So unless you want to go to one of the end-of-game area's, and farm it for probably 40+ hours, you will need to start PT2.
And even the end-of-game area's will probably start giving 1-10xp per kill once you are around Level 40.
After all sidequests/mainquests are completed though, you should be around Level 35. Seems to be the level most people end up on, after completing everything.