I probably wouldn't go with a Z4, but I'd definately go with the Hustler brand. Have you taken a look at the FasTrak Super Duty? They are very nice mowers IMO. And, the now come standard with ROPS (if that makes any difference to you).
'09 Hustler Super Z 60" 31 hp Kawasaki, Echo SRM-210 trimmer, Redmax HBZ 2601 blower, '98 Massey Ferguson 231 utility tractor, 5' Bush Hog 285 rotary cutter, 5.5' King Kutter box blade, 6.5' United disc harrow, United middle buster, Dearborn single row cultivator, single row HD row hippers, Bark Buster pto log splitter, log skidding "tongs", 6' Fred Cain landscape rake, LMC boom pole, drawbar, '09 Honda Foreman 500 4x4 FPE, '04 Chevrolet Silverado 1500 LS