Mercedes specifies premium fuel for the 103 engine, but the compression ratios are not that high. My 2.6 is speced at 9.2:1 and modern cars of this CR usually only require regular grade fuel.
Contrary to what you said, the 103 is a rather high revving engine with a torque peak in the range 4400-4600 and a power peak in the range of 5600-5800 with little rolloff to the rev limiter. You can feel this SOTP as the engine pulls most strongly from 5000-up and is noticeably soggy down low.
Detonation is primarily a low speed phenomenon. It is most likely to occur with high throttle openings and low revs. I've successfully used regular in my 2.6, but have to modify my driving habits a bit - primarily shifing at higher revs and avoid high loading under 2000 revs (I have a stick shift.) An automatic with a torque converter will have less tendency to load the engine at low revs.
There is a good chance that your engine might operate successfully on regular or at least mid grade. To determine whether it will, let the tank go to near empty, then pump in about three gallons of mid-grade. Drive the car and listen for detonation. Having the driver's window down will help as the sound will bounce off parked cars and city buildings. If it operates satisfactorily, try 3 gallons of regular when it hits empty again. If it detonates add 3 gallons of premium and then try mid-grade again. Keep testing until you have determined what it takes to avoid significant detonation.
Detonation characteristics are affected by ambient temperature and pressure, humidity, your driving style and driving environment (hilly or flat terrain). Because of of all the interacting variables, the only way one can know whether lower grade fuel will work is to test.
Modern cars with knock sensors (103s don't have them) will operate on regular grade fuel regardless of compression ratio. Lower grade fuel will result in less spark timing, which will reduce torque, particularly down low, but most drivers can't tell the difference.