Well I use my x749 for landscape work all the time and haven't had any problems with it. The pto is a quick diy install took about 30 minutes to do, 3PH took longer but had it on with no problems in about 3 hours, could do it again in half that time. I till, smooth, rake, seed, and roll with this on small yards often.
My suggestion to have a landscaper/farmer/neighbor run a landplane across the pastures would do alot to smooth up the bumps and rough edges. That is what I would do in your shoes. I would likely till up the 15,000 sq. ft and plant grass myself in lieu of sod, just completed a similar sized job with the x749 and it is in the watering mode now, took about three days by myself. Brought the backhoe with me but never unloaded it from the trailer, small machine did it all.
Another suggestion I would make looking at your property is to consider buying or building some type of drag or 3PH harrow to help with smoothing. A few passes over the ground each year when you had the time could really help it out. throwing out some grass seeds as needed. For your open spaces a simple drag type 5' section harrow would work well and you could pull it when you mow the pasture in most cases.
to answer your question about longevity, the x748 is built pretty durable and the 7 iron decks are stout, these should last a long time with your mowing. The new models that will be introduced next year are lighter guage but have the advantage of auto connect from what I have read. In your case where the mower deck will stay on the machine most of the time I would get the existing model with the heavier 7 iron deck and giturdone.
Heres a picture of my lawn that I ran my larger land plane across, really helps to get a smooth finish. It will save you time everytime you mow. Also pictures of the 3PH 8ft harrow I made for $250. Last is a picture of my small landplane a for the 749.