If you want battery powered, get an mpc 500, or a roloand sp 404sx. Now if you want a little more power get a mpc 1000, or better yet an elektron machinedrum uw (this unit will change the way you think about making beats), you'd need a battery pack to use them on the go though. Or if you have a laptop, you could use mashine, although you do sacrifice some portability.
Can't go wrong with any of these. What's your budget?
i would say an mpc1000 would fit that budget. as far as being mobile, if you wanna use it in a car you can get a power inverter that plugs into the cigarette lighter to run it.