You will need a mower that is in the "estate" or "light commercial" range to get a new ZTR in the 4 to 5 K price.
Check out the Husqvarna MZ 5225 or the Dixon Ultra 52. They are the same mower.
Others. The Bob Cat Fast Cat RZ series, Ariens Zoom series, Also check out Gravely GT series (Ariens sister mower).
Have you checked out the mowers for sale on this site? You can also check craigslist and ebay for used (and truly commercial) ZTRs.