I have owned a 10 ton 123 and absolutely loved the truck, it is about as BA as a truck can get! With that said there are a few drawbacks to owning a 123. Parts availability can be a real challenge especially since Fiat bought MACK, for certain parts your going to end up going direct to Fiat in Italy, they are the only ones that have certain parts left. Its going to be expensive in every direction! The top speed according to the TM is 38 MPH and the fuel mileage (if you can call it that) is .75 MPG. The weight of the truck is rather extreme of a 10 ton rated truck at around 37K pounds. Then you have the over width issue which will cause you some problems, (probably a lot)! In my experience the 123 is a absolutely beautiful truck in every way until you own one! The beauty is rather short lived. My first question is what are you pulling that needs such a big tractor? Have you considered a 939 series tractor? If thats not big enough look at a 916 or something in that series, THEY do have a road speed and are a fairly modern truck. Then there is the M920, it can also pull a quarter section of land if you can get it hooked up, it also has a good road speed and a pusher axle if your really going heavy. The CAT 7155 trans takes a little getting used to but after a few miles things start to fall into place. Like I said before the 123 is one of the most awesome trucks on the planet just unfortunately kind of worthless unless your moving houses!
Goof luck
Karl