If you normally use a pony book it on day one
Depends on how much diving you do and how you feel the cold but I use a 5m semi dry pretty much all the time in the red sea. The rest of the time I use a dry suit. I don’t ever remember being too warm apart from kitting up but I have been cold plenty of times
As far as kit is concerned, take what you can carry. Don’t bother with primary reels and stuff like that but do take an SMB and a small back up reel or spool if you have one. Surfacing amongst 20 livaboards on the Thistlegorm because you missed the shot will be a lot safer with an SMB
And if you get lost the boat will have a better chance of spotting you.
I take spare fin straps spare mask spare computer and every thing else apart from my tanks. Gloves are often banned when reef diving but your allowed them on wrecks. If your diving a lot, take ear calm and swim ear and use it every day. Sea sick pills, plasters, Sinutab are also handy if on a livaboard. I take dioreah tablets headache tablets and antibiotics as well.
Have fun
ATB
Mark Chase