DC Brushless Motor For Electric Rover
This is a bit off-the-cuff mathematically just to see if things stack-up: -

Your mass is 100 kg and your top speed is 20 kmph (5.56 m/s).

You want to accelerate at 0.4 m/\$s^2\$ - this means it'll take about 14 seconds to get to full speed.

The energy attained by 100 kg at 5.56 m/s is \$\dfrac{mV^2}{2}\$ = 1546 J and this equates to a power during acceleration of \$\dfrac{1546}{14}\$ = 110 watts or about 28 watts per motor.


  Am I looking for something feasible by a dc brushless motor?


It definitely sounds feasible - I don't see why you shouldn't find a brushless motor like this but it'll be easier to find a brushed one for sure.


  What Voltage/Amperage values am I to expect from a motor of these
  characteristics?


I think you can be looking at 80% efficiency so each motor will consume about 35 watts. 4 x 12V, 3A full-load rated motor should work but maybe I'd play conservative and go for something like 50 watts: -



I know these aren't cheap but it was virtually the first hit when I searched for "50W brushless motor". There should be plenty that are a bit cheaper.

Try this too - maxon are the supplier.