Buying IEMs for monitoring a band that doesn't even exist yet, still less have any gigs, is really putting the cart before the horse. It's great that you're seeking opinions and doing research, but for goodness' sake don't drop hundreds (or thousands) of dollars on a system that will sit unused in your closet.
What goes into your IEM is not really a function of the IEM itself - it comes from the mixing board. Boards vary in how many monitor mixes they are capable of - a big expensive board can give everyone their own, while a cheap board is only going to handle one or two monitor sends. Those can go to wedges or IEMs, but it's the same mixes. It's just a question of whether you're getting whatever the mix is from a speaker on the floor in front of you or straight into a headphone. The advantage of an IEM is not that it gives you your own mix - the board does that - but that it allows you to control the volume and block out the rest of the sound in the room, which will be a mercy to your hearing over the long run.
As far as what should go into your monitor mix - #1 priority is your own vocals, if you're singing. It's very hard to stay on pitch when you can't hear yourself. In a loud show, you can literally be shouting and still not hear your own voice. Little secret, though; if you wear a pair of $15 musician's earplugs, they will cut the volume of the room while keeping it clear (even frequency cut), but since you hear your own voice through the bones of your skull rather than the air, you will still be able to hear your own pitch.
If you're also playing guitar, you will want to hear yourself - there the question will be if you need the monitor to do that. If your amp is loud enough and the isolation of the headphones is not too great, you may not need it in the mix.
The other major priority would be that you need to hear the drums. That's what will keep you on the beat. Drummers are effectively the conductors of rock bands, holding everyone together. Depending on the gig, the drums may not be mic'd at all, or just the kick drum, or the whole kit (also depends on how many feeds the mixing board can take...). If they are mic'd, you can have it in your monitor. Again, depends on how isolating your headphones are to decide if you need it or not. Some people take their cues more off the kick drum, others more off the snare.
As for everything else - bass, other guitars, etc. - it's really up to you. Personally I would find it weird to be performing and not hear the rest of the band at all. If you're singing lead, backing vocals can be more distracting to hear than useful - you can get drawn off to their harmonies rather than staying on melody. Everyone else needs to hear the melody to harmonize to, but not so much vice versa.
Bear in mind that what you hear in the monitors is rarely what the audience hears in the house. Obviously, if you have a custom monitor mix, they crowd is getting a different mix from you. Also, acoustics can vary the sound tremendously, and the acoustics are totally different from room to room and situation to situation. Just this weekend I played an outdoor party where the band (only) was under a tent. The tent roof caught our sound and reverbed it down on our heads. My bass sounded amazingly meaty and rich and I was in heaven. Step out from under the tent and it was a whole other story - much more subdued bass and the vocals cut through much clearer. Which is good - I don't think the people would have liked hearing my bass as dominant as I did. Took a lot of tweaking over the first few songs to get the outside-the-tent mix right.