Seeing the EXP boost and the new weapon/armour rewards from MSQ and Job quests.. its quite easy to hit Lv.50, I've seen players Lv.48 with a Green Leaf on their heads (Still fresh). As for the Lv.50 dungeons, you are required to run only the ones that are part of the MSQ, otherwise they all are side dungeons that are only required a clear if and only if you want to unlock high level roulette.