I might be getting a Yamaha PSR E423 and I was wondering if I would be able to use it as a MIDI keyboard? Like be able to play the software instruments thru the keyboard?
Yes it will work. You may need a MIDI interface for your Mac in order for it to work properly. It is possible that it has such an interface already built in but generally these keyboards do not.
I got mine on Ebay - although I hear some generic ones work fine - Just stay away from the M-Audio UNO as I heard some Yamaha owners had issues with it. I went with Yamaha's own.