Engine weights/Choices for W126 Powerplant

I searched and didn't really see much on this.......I have a 1980 Euro 280SE with an M110 K-Jet that had been sitting for a long time..I started it and it sounds grim..I suspect stuck valve vs piston top, knocks like it, compression is way low in one cyl so I have to tear into that engine (positives are high compression Euro engine and good cams), I have a spare USA market M110 (low compression pistons, US spec cams; but donor valves if I need them); I also have a spare Euro M110 head and intake manifold.

In moving the spare M110 it came to my attention that this engine is HEAVY! For a measly 2.8L this thing is a boat anchor! I love the DOHC and Euro specs, but..............and here's the rub...........
I have an excellent M103 3.0L SOHC complete running engine/trans from an '89 300SEL W126 that I can transplant; and I have an all-aluminum 3.2 SOHC V6 I can use. Assuming the M103 is a bolt-in and the 3.2L would be more time-consuming to adapt, which engine makes more sense ?:
M110 Euro-spec reincarnated from my spare pieces?,
M103 US-spec alive and running, low miles?, or,
3.2 alloy block/heads V6 transplant?.......

I'm leaning toward the M103 but I haven't pulled it yet from the donor car and don't know how its weight compares to the leaden M110 (the 3.2L is a featherweight; has to be far lighter than either I-6)..........input??
DAW