Wisdom is required only for determining what level your spirit shaman is able to cast. So if you want to cast spells from 9th level, you'll need 19 Wisdom. DCs are determined only by Charisma, so the higher you'll go, the harder your spells will be to resist. Remember that you'll have an option to raise the abilities as your character levels up, so you don't have to max your Wisdom at the start.

Ok, but wouldn't Druid be the better choice?

It depends what do you expect from your character. :) Spirit Shaman doesn't need to memorize spells. Instead he can cast these spontaneously like a Sorcerer. Druid functions more like a mage. He needs to memorize the spells he wants to cast and rest to make them available for use. With Spirit Shaman you can also cast more spells of the same type per day as the limit applies to all druid spells available on a certain spell level. Spirit Shaman has also some unique features. You can check them here: https://nwn2.fandom.com/wiki/Spirit_Shaman. If you're a novice player Druid might be a better idea, but it all goes to a matter of preference anyway. :)

I just did a playthrough and gave him a couple of levels of Barbarian, which I read is the preferred class of a Spirit Shaman. That allowed him to be better with many weapons, and I gave him Weapon Focus in rapiers, so he could use the Elemental's Ruin rapier.

If all else fails, you can always cheat with the console to rearrange his feats and skills as you see fit.