[Espeefan]

I can't offer much advice on the screw drives, but I can offer some advice on the gearmotors you want to use for the drive wheels.

I think you'll find 150 RPM gearmotors to fast for the loader. With a 5" diameter tire, the loader will be covering a little better then 15" of ground, per one revoltion of the tire. Multiply that by 150 RPM (full throttle on 12 volts), and your loader is going to be covering 2,250" per minute, or 187.5 feet. That's pretty fast. About 2.13 actual miles per hour. Or looked at another way, at full throttle, it will cover 37.5" per second. That seems a little on the high side.

On a 7.2 volt pack, the same gearmotor should do about 90 RPM.
1,350" per minute, roughly 1.2 mph. Or 22.5" per second. Better, but still a little on the high side, at least in my opinion.

If it was my project, I'd suggest going for a something around the 30 to 50 RPM gearmotor range, on 12 volts.