tkamiya and I are talking about the same thing.
You should be able to rent the pressure tester, if you can't, don't bother at this point, just sell..
All you need to do is deposit $75+tax ($81) with your local AutoZone and they'll let you borrow the Coolant Pressure Tester.
It comes in a black plastic case. Its not the same as the cheapo coolant tester.
Its really easy to use. Remove the radiator cap and twist on the tester. It has a manual pump on the other end. Pump, until the gauge reads 20psi. (check you manual) w124's should be around 20psi. It should stay at 20, mine drop to 19, probably because I didn't want to over pressurize the system. I didn't want to even push it to 21psi just to see. At this point look for leaks in the cooling system.
LOOK UNDER the car!! I found coolant driping from under my car, next to the transmission. Jacked the car up, secured it and went under. Followed the leak back to the inlet at the firewall, next to the fuse box area. Just a loose clamp, wew, also found my top radiator hose clamp to be loose to, my mistake. (over cautous of cracking the plastic)
He held at 19-20psi for more than 15min. Thats longer than you really need to determine whats wrong. The manual that comes with the Pressure Tester will also help you determine other issues like Head gasket failure.
I am also being really anal about the temp issue, but I also want to be safe on long trips to Vegas with the A/C on.
Well try it out, or just sell it.
Hope your getting another Mercedes??