BEFORE YOU READ THIS! Remeber, its been 3 years since i've taken physics lol.
The fact that is on wheels DOES factor in, but in your favor.
As far as the weight distrubition it does change several factors..
If you have 20lbs at 2 feet, thats 40ft/lbs of torque, along with 120ft/lbs, which is 160ft/lbs. Now as far as your wheel base, if you could provide that, along with its weight(and height) we could determine its ability to resist tipping.
Another key factor though is the size of the base. If the base itself is 4 feet wide, and 250lbs.. then you could literally take the 30lbs and 20lbs load and lean them off the base, and it would not tip over. The fact that it is on wheels is in your favor, because it would resist tipping, and would move forward. Infact, this is how modern SUV's avoid roll overs. If it detects that is leaning past X point it will apply brakes to turn the vehicle INTO the fall, and transfer the motion from side to side, to forward motion(where the suv is far less likely to tip).