I have a
b3030, loaded r4 tires (foamed) and the 72" mmm. I've mowed the lawn with it for 5 years.
Chuck, given what you've said, I'd recommend you get a Rear Finish Mower and a quick attach system (I strongly recommend Delta hook - it's expensive, but you get a skid steer quick attach in the back basically, its just easy)
Why? The MMM is a bit of a pain to set up. I've gotten pretty good at it - under 5 minutes on and off, but it requires some yanking back and forth to line it up, and the pto can be a pain to put on blind (I've gotten good at it, but it took years of practice). Rear attachments are much easier to put on and off - with the delta hook they literally take 30 seconds, most of which is climbing on and off the tractor to put on the PTO.
Mow quality is quite good, especially if the blades are kept sharp. I sharpen them with a flap disk in a grinder, as recd on here, and its simply amazing how fast they get sharpened. My 72" deck will scalp if mowed low, but mowing low is bad for grass, so I don't mind. I have to mow when it is DRY or the tires will tear things up - I am seriously thinking of getting another set of tires and rims just for mowing, unweighted turfs.
If you are going to leave the tractor set up all season as a really big lawnmower, I'd say the MMM is fine, and has some advantages for mowing - as does RFM. Which matters more will depend on your property, but in either case you can get the job done. Perhaps somebody else can do a better job of explaining what I mean.
I drive over the deck, btw, without adjusting anything. Do it on dirt not concrete. No problems with any of the wheels. Standing on deck is obviously no problem - frankly, I'd even stand on a little push mower deck, I wouldn't hesitate to stand on any tractor mower deck I've seen. I'm around 6'5 230lbs fyi
hope that is helpful