On the Power Trac PT425, if the engine dies, you ain't going nowhere! You have to open a bleed valve to bypass the pump. That is because there is no transmission. Just pumps. So you couldn't really consider it an HST like a conventional tractor.
Every true hydrostatic tractor that I've seen has a transmission with two or three ranges and a neutral. My IH2500B is hydrostatic and the owners manual says that you have to put the tractor in neutral to tow it. It also says that you need to run the engine at 1000RPMs to provide steering and transmission lubrication. If the engine cannot be started, you have to add 4 gallons of HYTRAN fluid to the transmission before towing to provide lubrcation. When the unit is repaired, you have to drain the tranny down to the full line on the tranny dipstick.
You also cannot start the engine, even if it is in neutral, unless you depress the hydrostatic bypass valve with your left heel before cranking. The fluid in the pump will prevent the engine from turning over fast enough to start.
What brand of tractor were you looking at? It would be interesting to see which make and model the dealer was talking about. Hope this helps. /forums/images/graemlins/laugh.gif