Just got done with your same dilemma.
After 2 years of thinking about this and driving myself almost crazy, I settled on a 54" Big Dog R754 model.
13 year old MTD just had to go. I may have gone a little large, but we have 2.25 acres to cut and this machine is well, well built.
What finished the deal for me is when I discovered that these are built by Excel Industries, which built the first ZTR made in 1966!
You owe it to yourself to look at them well before you buy. I got my R754 for $4800 + tax, but you could probably do with a C series for what you need to do. I'd guess somewhere about $3500.
Not as commercial duty as The Hustler line they build, but plenty sturdy for residential use.
Once we get accustomed to driving it, it should cut the mowing time from 4+ hours to maybe 1 1/2.
Residential and Commercial Zero Turn Radius Lawn Mowers and Tractors - Big Dog Mowers
Charles