At those speeds and RPMs, you are probably okay. If memory serves me correctly, your car redlines at approximately 4500 RPM. I wouldn't pull a boat at those speeds. Keep an eye on your temps and MAKE SURE YOU CHECK THE OIL LEVEL at least daily. With the thermostat and water pump, it is pretty straightforward if you follow the maintenance manual. I would also consider a cooling system flush while you have the thermostat out anyway. Russell at DieselGiant has a nice write up on doing a citrus flush. It helps out a ton. Otherwise, take your time and don't cut corners, that's how other stuff gets broken.
If your coolant is over 2yrs old or if it's all nasty go ahead and flush the system. Be sure to roll the heater wheel to full heat that way you open up the heater core and it can be flushed out. Put the front up on ramps that way you'll have an easier time getting all the air out of the system all said and done. Crack open the bolt on the block and flush there as well. A lot of people forget to do that. It's on the passenger side of the engine. Have a look at the freeze plugs too while you're there.
I had the same concerns about engine speeds when I bought my first Mercedes diesel. It seems like they rev very high, but if you look into the shift patterns and normal shift speeds/rpms for each gear you will discover it is fine.
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