I will be using them for practice and gigs. We have a PA that we use for practice, as well as venues that do not have a PA. I imagine I would get a feed from the PA. So beyond the molds, what would I need (i.e., a mixer?).The first question is, what will you be using them for? There are various levels of quality, coordination, and expense involved with monitors.
Proper in-ears are custom molded for high isolation, and those costs vary widely, usually by how many elements are in each earpiece (compare to the woofer, midrange and tweeter in a speaker enclosure.) Most in-ears today have at least 2 elements.
There are Shure and other so-called in-ears that are little more than expensive ear buds with a foam piece that serves as partial isolation.
Isolation is important, because it governs how effective your mix is. In other words, it's just as important to block certain sounds, as it is to hear them, in order to control the mix. A good mix is absolutely crucial to a good performance and comfort level, so do consider isolation as a factor when choosing in-ears.
And by "system", do you mean the transmitter & receiver, or are you plugging into the existing hardware? (mixer, existing receiver/beltpack?)
Most musicians who need in-ears own just the molds themselves, not any other related gear (unless they happen to have ownership the p.a.) so hopefully, you'll just need to invest in the molds (in ears.)
More info about your specific needs will help with the answers you seek.