In general, "overrappelz-item.com/tag/level/">leveling" your armor/costume and weapon are going to help you. While you do not receive the full level bonus, you do get slightly higher adds to your derived stats. On the other hand, if you are trying to save money, you may want to avoid leveling your boots, gloves, and (non-CS) helmet since each level here adds very little to your PDef.
Another option to consider is to obtain boots, gloves, and helmet every other rank and level them to 10 soon after purchase . That allows you to spread the cost over more levels and provides much of the same protection. Note that leveling a belt increases the amount of weight you can carry, so that is something to consider when trying to decide whether or not to level it. Also note that when leveling a belt, for most, each level adds 0.5 to your PDef.