good choice. when i purchased 7 yrs ago, the price difference between the 7040 & 8540 was not that great. went with the ladder, a lot more tractor. not sure how that plays out now. definitely would get the rear tires loaded, at least 2 remotes, maybe 3, and the heavy duty trans. if offered on that model. to me, a hyd top link would also be a must. ....if you go with an M series, you're really going to see a difference over your MX, that said, the MX series is an excellent one serving many needs. best regards
footnote: i don't subscribe to the old rule of 5 pto hp per foot of rotary cutter, esp in a commercial application as yours...to me, 8 hp would be more accurate. in the field your knives will dull, there will be tall growth, & you will want to maintain speed to make money if mowing is your business. the 5 hp rule just won't hack it. but that's just my bias