I bought my 300se about 4 years ago @89k. Now at 97k.
A/c Was expensive, about 2.5k, water pump, and regular maintenence.
So, I guess about 1k a year for it.
My 300e, (197k)which has been kicking and screaming to the 200k mark, has got me for about 1.5k just this year. (rear subframe)
And, I just found out my evap started to leak exactly 1 year after I changed everything in the system except that.
My opinion is that the m103/w124/126 starts to get real expensive to keep after 150k.