I dont have massive experience with pedal boards but I can say:
Unlikely to switch channels on an amp as that (unless its a midi controlled amp) requires a latching switch which I've not heard of in a mulltifx board. It's not usually too bad hitting 2 pedals when changing sounds even live..some guitarists have gotten it to a tap dancing art! :)
You get what you pay for - a multi fx will have compression/noise gates, but its not going to be as good as a stand alone effect, same as distortion/overdrive. These are generally better as analogue effects.
Yes, with a multi fx if you set the amp clean you could then set a preset on the unit for clean, and another for dirty each having its own combination of effects (reverb, delay, chorus etc). And with most of these units you could have hundreds of such presets all different, so they are super flexible at the touch of a button. So great for adding some different icings onto whatever your preprepared cake (basic sound) is/are.
As Ailean suggested - go have a play, see what you think. :)