It's cheaper to re-roll rares than legendaries, so roll the enchants before applying a legendary affix
Helltides are the only drop locations for the mats you need to re-roll enchants
--Helltides.com has got a tracker for when they'll be and where the Mystery chest is.
You can do the capstones at any level, but if you're early it could be tough.
Level on World Tier 1 the first time, the XP and gold bonuses don't equal the increased monster health so you'll level more slowly even though you get more XP