Do you want a hard-wired system or wireless? Wireless gives more freedom of movement and need not be expensive. Wired probably gives better sound quality for the money, especially with a headphone amp.

You will want some control over the volume. With both wired and wireless the beltpack should give you volume control.

Using in-ear, even in one ear only, will reduce or muffle some of the sound you hear acoustically. Although your objective is to hear yourself better, you may find that you also want some of the rest of the band in the mix in order to hear everything clearly. If you are working against loud instruments such as pipes it may be an advantage to use well-fitting phones in both ears to reduce the volume of what you hear directly and have a more controlled sound in the foldback mix.

A good alternative to in-ear is a personal monitor mounted on a mic stand, for example: Behringer Eurolive B205D Active 150W PA/Monitor (I've no experience of this particular model, this is to illustrate what I mean)

How many are in the band? You may find that you all play better by investing in a bit more gear for the whole band, which would allow everyone to have the foldback mix which suits them.