I'm going to second
's comment on URO parts. They are cheap Chinese, and will cause you heartbreak. As for the connector, it's a critical part, and I'd be inclined to go OEM also. You won't be paying much of a premium.
Also, you don't need to replace the lock pin on the dipstick tube. I've run my '99 C280 without the lock pin for 25,000 miles, and it has never come off, nor leaked, nor done anything untoward. And having the pin out lets me check the fluid level more easily.
As for your fluid, it looks bad to me. Yeah, the grit is what you say, but that oil should be pink-ish in color. So do the fluid change ASAP.
When I changed the fluid in my '99 C280, I had to disconnect the exhaust pipe which crosses just under the torque converter and then put a screwdriver into the joint. This deflected the exhaust tube just enough to allow me to get at the drain plug for the torque converter. I'd highly recommend draining everything, including the torque converter. That way you have the cleanest fluid possible going forward, and given how dirty your fluid looks, I'd consider that important if it were my car.
Finally, once you've drained the fluid, look at the bottom of the pan. Some clutch dust/residue is normal, but if you see shiny bits of metal, you've got a more serious problem. Post a photo or two, maybe one where you've dragged a finger through the thickest bit.
Finally, there's a terrific thread here on the 722.6 transmission. I'd read it, and also look up the Sonnax spring replacement kits. Here's a link to all their 722.6 transmission products. https://www.sonnax.com/units/211-722-6
The regulating pressure control valve spring is a common problem, and if you're in there, it might be wise to simply replace it. It's about a $10 part.
Good luck and let us know how it turns out!
Where we recently had a weird problem with our own transmission that appeared after sitting for 2 months, and then went away after being driven for a few days. I suspect it was the shift position sensor in the console.
P.S. My transmission (with the torque converter) took about 7.5 quarts of fluid, if I remember correctly.