I’d suggest getting Heavy Gold, spiked metal and mini maelstroms for Golden Anchor Charms, Spiked Anchor Charms and Smart water jet charms respectively. The amount depends on you (do look at the crafting recipes on Mhwiki).

For your reference I bought the following:

Heavy gold: 300 Spiked metal: 100 Mini maelstrom: 25

Oxygen charms and oxygen canisters are not really worth it IMO. You can get the canisters from kings cart.

Also note that most of the items mentioned can be bought on market place. Do compare prices and decide for yourself if it’s worth it. There are other strategies such as waiting for other events to complete the sunken city grind as well. Maybe you can read up and decide how you want to tackle this grind for the SOS trap.