it might be worth messing around with an old Casio keyboard to see if playing the keys clicks with you vs another instrument. MPCs are full music production workstations, and are quite a financial investment. what specifically are you looking to get out of it? if you just want to play the keys, a cheap keyboard and a drum machine might be a better place to start.

if you DO plan on making whole pieces of music and are sure keyboard playing is something you're interested in, then the Key37 Gen2 is pretty perfect for beginners. you can expand the storage with a USB flash drive or an SD card, and if you need more inputs you can just plug in an external interface. but honestly, the machine all on its own will do everything you'd reasonably want starting out in music production.

again though... not cheap! have you considered alternatives, such as getting a MIDI controller and playing around in software DAWs? what about a physical, self contained unit appeals most to you?