The ZL has a softer suspension than the ZR's. The standard track is a 121", so they must have done a track/tunnel extension. It should be an awesome trail sled. I wouldn't do a lot of fast ditch banging with it though, with the softer suspension.
The 600's aren't the greatest on gas (my wife's gets about 8 mpg -- Imperial gallons in Canada), and are heavy on oil but I'd rather spend an extra $50 a year on a couple more jugs than do an engine rebuild.
Should be a good sled if its been maintained. Good Luck!!!