I will be using the applications for future projects and am expected to include all current functionality and to improve upon this. I have two challenges:

1. The existing code is not OOP, so using it as a library upon which to build more functionality is extremely difficult. Changing function X to do Y will break function Z.

2.
Isn't anyone at your company using them? Hopefully, they would know what they are supposed to do.
The applications were used by previous customers or the developers themselves. The previous "group" was pretty much independent. Thus, only a few people here know what they were used for, but none know how they actually did the task or how to use the application.