I'd got for the 104 engine over the 103 having driven both- the 104 is much more fun. The 103 seems slightly underpowered and lazy for the big heavy 124.
-that said, the 103 is easy and relatively cheap and simple to work on.

I think the build quality was probably better on the earlier cars though.

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1993 320TE M104
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past:

1983 230E W123 M102
1994 E300D S124 OM606 (x2)
1967 250SE W108 M129
1972 280se 3.5 W108 M116
1980 280SE W116 M110
1980 350SE W116 M116
1992 300E W124 M103
1994 E280 W124 M104
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"music and women I cannot but give way to, whatever my business" -Pepys