The thermostat is very easy to replace on the 103 motor.

3 -> 10mm bolts come out and off comes the t-stat cover. You're going to replace the upper hose that connects to the t-stat connection, so now would be a good time to do it. There's a jiggler valve or air bleed valve that needs to be in the highest positon when you lay the t-stat in the housing. There are basically 2 brands used: Behr and Wahler.

Behr is considered OEM and Wahler has received bad press here. I've used both and have found the vehicle to run cooler with the Wahler. Your choice, though you're more likely to get a Behr unit if purchased from a dealer.

Some will say why not change coolant while you're at it? That again is your choice, but if you elect to go this route, buy the coolant from an MB dealer and mix with distilled water -> 50/50.

The radiator on your car has a red colored drain plug that in the lower front part - passenger side. In some cases this drain plug is very hard to get to. It is in mine and as a result, I simply remove a lower hose(an inch or so in dia.) from the back side of the radiator - passenger side and let the rad. and overflow tank drain at the same time. The block is drained by loosening a 19mm metal drain plug that's found on the passenger side in between cyl 5 & 6. You can hook up some heater hose to it in order to catch the draining coolant into a pan.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"