First thing I want to clarify: there's no "hacking" needed to do anything in MODX. And definitely not for a directory listing. There'll be custom development, yes, but you wont be hacking the core.
A "directory" listing is very broad and different situations would mean different recommendations. If it's going to be a directory listing that may not ever go over a couple hundred listings; you might be fine with using plain resources and tools like getResources, tagLister and NewsPublisher. If it fits your requirements, bdListings might be useful as well. You could even use Articles for a basic directory listing.
If you're going in this big time and are aiming to get thousands or even millions of listings over time; you'll definitely need to think custom coding. Wether using MODX or a different existing tool makes more sense depends on your exact requirements.. the basics of a directory listing script aren't hard to build, but it could make sense to use what others have done as a start if you are low on resources or just want to borrow their expertise.
If you could let us in a little more on what you're trying to achieve, I might have some more concrete pointers, but right now there's a lot of question marks on what your definition of a directory listing is