Battlewarrior Good suggestion. I would only add to it that having 15,000 Sx is never a problem for me. The only other item I ever buy is a Durge Sword for Simon. It helps considerably in his battles in the the Orgasmic Empire in his part of the Divided Party. No need for such for Nalili. Her Sexy Blade is quite effective when using her attack skills.
As for the Freezing Staff it remains highly useful at least up until you get two new staffs after the Erosian War when you find one in Stineford Mines and another in Deepwilds Shrine. It never goes to waste, as Riala can use a staff as well as things like Elegant Whip.
But as far as purchases go, one item that should be purchased as soon as possible, (first trip to Stineford), is to buy two buttplugs, one for Yarra and one for Qum. You should also get one ASAP for Nalili, as it makes her skills immediately useful.
One thing that helps the spider battles in Stineford greatly is to buy ONE steel shield for Hilstara. Shield Wall immediately becomes much more powerful and effective.