Hi,
as a returning player I assume you have things such as mounts, gliders, etc. Aside from that you can get started with the returning player pack. It offers some basic gear.
For more info on how to improve your gear, check out
this page. They made it a lot easier to make the gear so that you basically only grind for infusions and scrolls.
Your biggest hurdle would be your gems, as the old system got wiped and you'd have to invest in those. At a rough estimate/guess, it would maybe cost at least 120k gold to straight up buy the T3 gems you need, excluding expensive endgame gems. At current rates, that would be about 30-35k credits.
Other than that, it is popular to invest in one erenor weapon to go alongside hiram/dungeon gear, and a maxed out eternal can go from anywhere between 500-750k gold depending on demand, gems, temper, etc. This would be 125k-190k credits.
Hope this helps with your estimation of how much you'd wanna invest.