Okay- my two pennies
Reg- The only thing you HAVE to have underwater? Air. Your gear, most important IMO
Computer- ascent times, NDLs, surface interval, it accounts for all of it. And rentals can be dicey.
BCD- Face it, you Can wear any bcd, but they are like underwear- yours will fit the best, and you can sure of how it has been treated and cared for....
Emergency gear- did I need to bring up the smb, mirror, whistle... all that?
DAN insurance- seriously, have some.
Random bits and bobbles...
good diving.
And don't stress about this too much. You'll accumulate it. We all do. As long as the kid's college money holds out, that is...