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.
I'm leveling a sorc now as a casual player on world tier 2. There's a lot of versatility on leveling builds. Change things up often and experiment. I found a legendary that put frost nova on 2 charges instead of 1 and it's been a blast teleporting into a group, frost nova, and ice shard machine gunning down everyone. Before that I played around with lightning builds and they were pretty fun too.