If you can afford it, do not return the Motif. One question, do you own a computer and if so what soundcard and sofware do you have? In the meantime...
Basically, if you DO have a computer and soundcard your going to need some kind of keybaord to play via MIDI into the MPC. The MPC can then play the sounds from any synths you own, so if you had alot of softsynths that would be great and you might be able to sell the Motif and just get a basic MIDI keyboard as you wont be relying on its sounds. Also, If you have a computer you can slave the computer to the MPC and then mix in software, or even use a software sequencer for your melody stuff and keep the MPC for only drums and samples.
However, if you dont have softsynths or a computer, you really need to keep the Motif to give you your bread and butter sounds basses, pads, strings, guitars, leads, even extra drums which you could blend with your internal drums on the MPC. To do this you will need a mixer, although if you had a computer and a multiple in soundcard you could mix it all in software...arghh...
you really are in the end going to need a computer to track your beats or at least a harddisk recorder... Multitrack recording is better because it allows you to tweak your instruments and drums and create a mix. So, you could get a hardware multitracker or a PC. With a pc you then get some software, I use Cubase to track beats in. That alone will work, however computers are the best sound sources, so for me, I use reason, stylus, trilogy, FM7, B4 plus a bunch of effects, my favorite at the moment being "vintage warmer"...but I digress.
hope some of that helps...BTW, you can slave your MPC to the Motif, or visa versa. It sounds like your very close to being able to make killer tracks, presuming you have talent, are willing to work and keep reading these forums!
good luck
Mike
p.s. Im a bit tired, hoped that made sense