You don't need to know how to read notation for a band, but you do want to be able to read basic chord charts. This is the language used by most rock/pop/metal/country/folk and even jazz 90% of the time. Without some sort of common language, it will be very difficult to communicate material to bandmates or learn cover tunes from tab. Learning chord charts will require learning a little basic theory, but that comes with the territory. For example, you'll want to know what a Dm chord is, and in what voicings (fingerings) you can play it.
The one thing to consider, is you should probably learn A LITTLE notation along the way, even if you can't do it in real time. Often, keyboardists are called on to do melody and lead lines, which have to be presented in notation, unlike rhythm parts (chord symbols).
All said-and-done, learning by ear is the most important. Even classically trained musicians (myself included), play by ear most of the time. Notation is just an artificial map. It's good to think with your ears, so whatever method works for you to learn the material, not gonna judge!
As for gear, what kind of music and material are you hoping to play? In most bands outside of purely electronic music, you'd be surprised at how much tried-and-true triumvirate of [piano, electric piano, and rock organ] are used. I'm on one of those sounds about 80% of of the time, no matter the genre. They blend well with all other band instruments, they're traditional and don't sound out of place, and the most versatile sounds that a keyboard player can use. So make sure, whatever you get, has good solid basic keyboard sounds. Those are the most important, bar-none, so get comfortable with pianos first, and "rock" organs next if your genre uses them (most can). Next up are the two synthesizer sound categories: Pads (for filling background space), and Leads (for playing melodies and solos). Synthesizers are fairly genre specific. You won't hear them in acoustic styles, country, classic punk or grunge, though all of those can include piano/organ.
I recommend something simple and cheap to start off with. Skip a synthesizer or workstation, go for a 61key "keyboard" with a wide range of sounds that you can just switch to and play. Don't worry about more complicate instruments with sound creation engines (synthesizers), at this point in your learning, it's good to be un-incumbered by non-performance features, so you can concentrate on putting fingers to keys. I can't recommend a specific brand or model, since they change all the time and I'm not personally in the market for something like that.
Finally, playing in a band, you're gonna need an amp. Many smaller cheap keyboards like what you're looking for will have builtin speakers, but they'll NEVER provide the volume to play with other musicians. So, don't worry about the quality of built-in speakers and get yourself an inexpensive external keyboard amp (but NOT a guitar amp, those are different).
Should be able to find a small, simple keyboard for $150-$300. Small keyboard amp for $100-$150, and stand $25. So you're looking at anywhere between $250-$500 for everything, maybe even less if you buy used or get a good deal. It's a shame you're looking now, holiday sales tend to have TONS of discounts on cheap music gear.