Changing fuel filter
 

How hard is this to do, and are there any special tools required?

In other cars I've done this to it was a simple case of taking it out from the engine bay, but I noticed in the 124 it's under the rear side of the car for some reason

It's been "under the rear side of the car" on German cars for decades.

If it's like my model, there are no special tools required beyond the 1/4" drive extension needed for the socket to remove the plastic cover under the car.

I think you'll just need a couple rather large open-end wrenches. I believe you'll need a 22mm, 21mm, and then I think the other side will be the normal sizes, 17mm, 19mm, in that range. But nothing really special, other than the wrench sizes on the banjo fitting being a bit large.

Is there anything needed there to stop fuel leaking out?
Pressure release?
Clamps etc?

Or is it as simple as unbolt and bolt?

You won't lose a terrible amount of fuel, mostly a portion of what's in the filter and some pressure leak. You can loosen then retighten the fuel cap, which will help if there is any pressure in the tank.
There were a few variations from model to model, but I don't believe yours used any clamps. If so I would reuse them, MB uses excellent hardware. You will need to loosen the screws on the strap on each side of the filter, usually not a problem. You'll need a couple copper washers for the banjo fitting. You wil get a small amount of fuel when you loosen the first fitting, due to pressure in the lines. Protect your eyes at all times and wear ratty clothing.
HTH
Gilly

PS I usually loosen the strap that holds the filter in place FIRST, so it is ready to be removed once the hoses are off. You will need 2 wrenches when removing or installing the hoses/pipes to the filter, one for the fastener and one to counterhold on the filter side.

you will absolutely need a couple of the small aluminum washers that go between the fittings and the filter. These always leak if you try to reuse them.

DG

Thanks guys I'll go to the local MB and pick up the stuff I need

The rings should be copper though, not aluminum. See what the dealers sells you.

Just did my filer on a 91 560 SEL. I had to re-use my rings. they were copper and needed a good tightening to prevent leaking.

On the other end I used some teflon tape on the threads. I had an issue though. The filter came with a fitting that I had to remove because the one on the fuel line on the driver's side was fused/rusted onto the fitting. So when I removed the filter, that fitting stayed.

So I removed the bolt-but thingy on the passenger side with the TWO copper rings. I think unthreaded the filter from the line on the driver side. Not that I did it, I am sure next time will take a lot less than an hour!

If you anneal the washers with a Bernz-O-Matic then sand them on a flat piece of glass and fine sandpaper you should be able to reuse them just fine, but that's a lot of trouble so just get some new ones and be done with it. Damned if I could find any when I replaced the fuel filters on my car though.

The other thing I was thinking of doing along with the fuel filter change is removing the injectors and having them ultrasonically cleaned.
Does this seem like a good idea?
And, any write up on how to do this?
As far as I can recall, the last time I removed injectors on a car it was fairly simple, with the only thing to note was to put some grease around the rubber o-ring to make sure no fuel comes out.
Can anyone comment?

I replaced the check valve in mine also while i had pressure off the system since its right there.

NO leave your injectors alone.