To echo Elau, if you plan on buying new and keeping it, most definitely get an extended warranty in addition to the 4 year/50K mile std. warranty or get a ML with a Starmark warranty. From previous experience, I wouldn't own a MB without a factory warranty.
Also, re test drives, my dealer (where I bought my previous MB) let me evaluate a low mileage '98 320E 4-matic wagon for about a week and then a brand new ML-320 (it had 18 miles on the odo when I got it) for another week. The point being that the dealer should give you plenty of time to decide….. As one last point on the value of the extended warranty, the service technician I work with told me that the MCS unit they replaced is a $5,000.00 item. On my ML that item & most everything else will be warranted for 100K miles.