So that could vary from just a DI box to tone shaping and effects to all of that and I want in ear.
However, none of that in essence is difficult. If you like the sound of your bass and need nothing else, buy a DI. If you want tone shaping you'll need a Zoom/Stomp/pedals then DI. If you want one of those and IEM then you'll need two outputs, DI and IEM, and it's more complicated and more money.
Btw, a cheap and reliable way to do iem.. instead of using a wireless system (expensive, flakey, black magic), I use a wired system (cheap, reliable not voodoo). I run a physical cable to a headphone amp on my belt to my in ears.