You must acquire all 103 pieces of equipment/costumes. This can be viewed by pressing , then scroll over to the Collectibles tab, then scroll over to Equipment. Here you can see your status. The costume sets/equipment are acquired from a combination of looting them from drawers, bags, etc., looting them from knocked out bodies, getting them as quest rewards, and buying them from vendors. Note that, for example, the Witch Hat, Witch Robes and Witch Gloves do not count as 3 toward the 103 counter; you must collect all three pieces of a single costume to count as 1 toward the 103 counter, so don't be surprised if you collect a hat, robes or gloves and don't see your counter go up by 1.
Here is a list of all of the costume pieces and equipment you must acquire: http://www.videogamesblogger.com/2014/03/07/south-park-the-stick-of-truth-weapons-and-costumes-locations-guide.htm
(Full credit to the guys listed at the bottom of that guide for assembling this perfect list)
The equipment you can/must buy from vendors is NOT missable (you can return to every vendor post-game to buy everything they're selling), however some of the looted equipment is. Be sure to follow the Missable Guide (linked) to make sure you at least pick up all the missable equipment during your playthrough.
NOTE that you need not possess every piece of equipment simultaneously! In other words, you CAN sell old stuff to make money to buy new stuff. As long as you've possessed the equipment at some point, it will tick off on your equipment list.
If you still need money to afford everything after selling all of your useless stuff, refer to This Post for some money farming methods.