Our local MB dealer still has a couple tech's who know the older cars, but the costs go up rapidly when its anything beyond a basic repair like an alignment or basic suspension work. We try to avoid them for anything but buying parts or alignments. We have some newer cars now (W210 and W203) and they will readily service those. Prices on some procedures like transmission fluid replacement are reasonable too.
In general getting work done on cars 30+ years old is going to get harder and harder.
Once I retire my 82 SD my oldest will be my '89 420.....which at the rate I drive it will last about 40 years before it is worn out.
My daily is my '93 190d and my wife's is a 2005 C240 wagon....both have a LOT of life left in them.
IMHO cars like the W123 are neat to have around "in the collection" but I don't consider them a viable daily driver anymore. W124/W201 are the oldest I'd go for a daily driver at this point. My 420SEL is my fun car....and I only drive it about 35% of the year.
The modern conveniences are also really nice....our W203 is amazing....solid, comfortable, reliable....all wheel drive...its pretty hard to beat. And I can troubleshoot it with my android tablet. Can't beat that.
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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)