Exhaust manifold leak

[Ben300SD]

I have suspected an exhaust leak in my 300D (W124) for quite some time now. The engine compartment likes to dirty itself even though the engine has no blow-by, and I can sometimes smell diesel exhaust in the cabin. Well, I think I finally found the leak. I started my car with the hood open the other day, and a nice little puff of smoke came off of the engine by the exhaust manifold.

Upon closer inspection I found that the leak is coming from where the manifold meets the head at either cylinder 4 or 5 (near the front of the car). The head has a nice layer of soot on it in that area.

I have several questions regarding this leak. First of all, how urgent is this repair? I am low on money at the moment, but if ignoring this repair now will cause more expensive repairs later I will bite the bullet and pull out the platinum card.

Second, how expensive is a repair like this? I want to explore all options before I take it to my mechanic.

Perhaps this is something I could fix myself down the road sometime, but right now it seems a little over my head.

Any advice?