choosing the suitable DC motors
Let's work out the basic physics of the situation (note this assumes the rotating mass is zero, a reasonable first approximation for this system):

F = ma

F = 0.5m/s^2 * 100Kg = 50N

50N / 4 wheels * r = 50 / 4 * 0.125 = 1.5625N-m (Thanks for checking my math Dave)

The circumference of the wheel is pi * 0.25 ~= 0.785m

20km/hr ~= 25478rph ~= 424rpm (shouldn't be a problem for a brushless motor)

So four brushless motors that have this torque in this speed range should nominally meet the requirements.  You would normally take a fairly large margin (e.g. x2).  If you want to drive uphill or over uneven terrain you need to consider gravity.

A good reference for the physics with handy calculators is:
http://hyperphysics.phy-astr.gsu.edu/hbase/hph.html