GENERALLY your AF gear should be okay. You'll wanna replace it as you go along for a few reasons, namely the HM primals have a bit more of an ilvl requirement. Also you'll wanna have ~3.2k HP for Titan HM to make sure you can survive a full set of Tumults and not get knocked out before a Medica catches you.
IIRC you can buy the i55 gear at the highest rank.
