Right. The merchant in China is a bit hard to find. You will find a long, narrow bldg. marked as the market in China. There are 3 merchants inside and none of them offer the items you need. The merchant you are looking for is in a non-marked bldg that is in the same complex. Just check the adjacent bldg's to the market and you will find the guy.
There is no "Portable Toilet"....the items you need to purchase before going exploring are a tent, Shower-in-a-Can and some dried food. That's all. If you need to use the restroom while deep inside a dark, dank tomb then you have to exit and find a restroom. Btw...the tent is re-usable. You can pick it up when you are done sleeping so you can use it again. The other items you should buy a few of because they are one time use items.
Not worth it anyway. Can just run back to base camp when your sim gets tired, it's not like the tomb resets, fruit is a heck of a lot cheaper than the dried rations, and being clean in a dungeon isn't really a high priority.
The dried rations have the Good/Great Meal moodlet, so they're not bad considering their price. You'll find plenty of high quality rations on your adventure though, so I've actually stockpiled some 40 just from treasure, so there's no real need to buy them in bulk.
I generally don't send my sims off on Adventures unless they have Steel Bladder, Dirt Defiant and Hardly Hungry, it makes it so you can focus on your sim getting solving puzzles and less about them wetting themselves.
The Sultan's Tabernacle is the best tent in the game and can be bought for some 2500 Ancient Coins and Visa 3, but if you spam the Moodlet Manager as much as I do, sleep becomes a foreign concept to your sims.