We just today went to look at a Ferris 3200 commercial zero turn with a 61" deck.  The only thing holding us back is the price of corn and the fact we still have over half of our beans to plant.  Ferris has a coil spring/shock suspension on all four corners.  Earlier NAT posters rate them highly.  We demoed an X-Mark of comparable size as they are VERY popular "here".  If we decide to buy, it will be the Ferris.  I believe the suspension will allow the deck to follow the ground contours more closely with less scalping - a problem we had with the X-Mark even though we were mowing at 3.5" (did I mention that our yard is not very level).  The X-Mark had a fancy suspension seat to smooth the ride for the operator.  Ferris's suspension smooths the ride for both the operator and the machine.  The 3200 is built like a tank and was priced at $11000; the 2100 is a bit less beefy and priced about $1200 lower.

Edit to add:  Top speed listed is 12 mph and you may be able to adjust it to get more.  I'd be tempted to skip the trailer as well.