Well,I don't wanna kill your idea but just a few facts about M911s......
The run 1400 X 24 tires,next to un attainable except by Uncle Sam,and then they are about $1000 ea for DOT approved ones,so finding bigger tires is gonna be a problem.
They suffer from the same problems as the previous/ and later models of tank transporters,slow top speed and unbelieveable low fuel economy(4 mpg empty).
They are appealing to house movers,logging companys,and other big businesses,which means they usually command top dollar($40,000 isn't uncommon).
Since they have low production numbers,most dealers don't stock parts for em reguarly,so special order parts are sometimes the norm(once again big $$$$$).
If you can't find wheels and tires to get it down to legal DOT width,you'll have to have a permit to drive it anywhere,and you'll have to follow specific routes.
If you ever want a trailer for it,you only have about 3 or 4 options if you keep the stock 5th wheel plate since it has an oversize king pin(3 1/2 in vs the standard commercial 2 in).
All that said,good luck finding tires or rims to fit and get it less than 103 in wide,lots of us M123 owners have been trying for a while with not much success,it's easier to pay the $100 per year for a blanket permit and be done with it.