You bring up a very valid point. The M1 weighs a freaking ton, and while the wheels are great, I can attest that they do not perform well over gravel.
That being said - I actually have a plastic deck (trex), and since the firebox on the M1 sits low, you do in fact need to take into consideration that the small area underneath the pit will get hot if you're smoking. I personally believe my solution to this is easy, cheap, and works incredibly well.
If you get the M1, pick up two pieces of hardie / backer board from Home Depot / Lowes. Whenever I want to use my M1 to smoke, I roll it directly on top of a piece of hardie board that I put on my deck, which takes all of the abuse of the heat and weight. The reason why I said to pick up two pieces is because if you ever want to move the M1 around your property, and it's all gravel, pop down one piece of board at a time and roll the M1 right over it (move it a few feet, then move the other board in front, repeat). Works really well for me.