The W123 is so cool, I have one too so I'd like a Hemi but in Europe byby. I like the solidity of the suspension and brakes, the geometry in rear, all is very solid.
The W109 were 6.3 with 4 spd auto 250hp
The W116 were 6.9 with 3spd auto 286hp
Actually there are W123 with column shifter, but the gearbox is Benz, so you can use the shifter fom W210 E class or CLK's
The ring & pinion will last if no neutral drops.
There is a Pro Drift car S600 in Hungary. Owner and channel name: Amon Oliver, sounds really unique, has a full cage, stripped interior, manual transmission, and the V12 is set back like a foot. They first came out with 408Ps / 402bhp then 394/388 for emissions.
When he drifts that along the 500E it looks a scale up, and the 500E looks one size up the other cars M3 and 200sx so the "whale" looks mighty.
The camaraderie in offroad racing comes from it's actual amateur roots, when several people with cars decide to see if they can pass something or climb a hill, and they must help and keep every car working, otherwise they all have to stop and recover and fix it.
Very interesting Mustang, You can build it in many ways but I am curious what you will choose. Would be cool to be competitive with a modern AMG in performance.