If you want to be able to access the keyswitches in the lower octave whilst you’re playing something in a higher register, such as violins in the Halion Symphonic Orchestra set, then an 88 key controller keyboard is the simplest thing layout-wise.
Of course if you’re not too worried about putting in the articulations as you play, you could use a shorter keyboard, press the octave down button, and record the keyswitches in a separate pass, or even paint them in in the key editor. A 61 key board means you don’t have to jig about too much.
The HSO sounds, and many others, use the Mod wheel to control internal note dynamics with sounds such as strings, brass and winds so often you find yourself needing 4 hands, or recording articulations and dynamics in separate passes.
Something with a few knobs and sliders will also allow you to easily access the quick controls of the HalionSonic SE and other VSTi s. Useful for instance if you’re doing an organ part and you want to alter the stops/drawbars/ rotary speaker settings sort of stuff as you play, to do swells and such. Or Moog type sounds where you want to crank the filter as you play.
A set of transport controls on the keyboard can be useful and means that you don’t have to continually turn to the computer keyboard to navigate around the project whilst you’re trying to play.
Think about what key action you want too. Springy synth action is fine for basic sounds and inputing drum parts or whatever but a semi weighted action feels more natural, and if you’re doing a lot of piano playing then a piano type hammer action is worth looking at.
One thing that has caught me out in the past with some budget keyboard controllers is the velocity curve response. You find yourself having to hit the keys really hard to get louder notes, making you have to go back and edit the velocities in the key editor. I’d look for something that has the ability to choose or alter a velocity response curve to suit your playing style.
Hope that helps. Maybe a real keyboard player (i’m primarily a guitarist too) would say something different of course!