Hmmm, with respect, I think you are a bit on the hopeful side.
To lift 500g to 1Kg on the forks, assuming a 'normal' forklift configuration, you must aim for a Kg or so right at the back, as ballast over the rear wheels. Some of that could be as batteries but your basic 120 x 100mm size may preclude large, lead-acid clunkers.
You don't indicate which motors do which jobs so we have no idea what currents to expect. However, if you use L298 (which are effectively dual, 2Amp H-bridges) at one-per-stepper and one for two DC-motors, you are using up processor-pins at a great rate.
Each L298 will need 4-signals, so that's 12 pins used (15 if you use 'enable' to reduce current-demands.) Pin 0 is not available to us mere mortals, 1, & 2 are best kept for interrupts, leaving P3 to P10 for say, 2 steppers. Use P11, 12, 13 plus one of the pins of the Analog port for another '298 (for the 2 DC motors) and you haven't got many pins available for your 4 to 6 sensors.
You might have to consider using a shift-register to reduce the pin-count, or perhaps not all the sensors are needed at the same time? Could they be paralleled?
Consider using the Screw Shield - you can stuff several wires into each socket. I find it very handy - in fact I have one pair pre-wired for a job that I set to one side. Can't remember what the job was now, but it seemed like a good idea at the time.