I'll try and attach some pictures.

I don't even think I've even changed a rear crankshaft seal on a C13 , yet. I had to look up parts diagram to see what design it was, it is like the C12 design. Look at the rear c/s seal and adapter plate, first. That adapter plate bolts directly to the engine block, THEN the oil pan bolts up, underneath of the adapter. SO, The rear flywheel housing(bellhousing) has no oil / crankcase pressure against it.
In your description, I would assume that you would be repairing/replacing both the adapter gasket, o-ring for the rear c/s seal and the rear c/s seal itself.
Now, If I remember correctly, You have to use some heavy grease to place the seal(big o-ring) onto the adapter, FIRST, then push on the rear c/s seal(by hand), loctite the little bolts(as they need to be sealed, too=not "blind" holes).
This is assuming the c/s wear surface is ok, yet.
 The new rear c/s lip seal should come with a plastic "installer" that slides the lip of the new rear seal, up and over, the crankshaft.

Now, for your oil filter base leak, I have done 3 of these "plate style" gaskets(between block and filter base), w/o pulling the oil cooler, BUT I was kinda lucky, too. That upper, rear corner bolt (IIRC) is a little up and under the oil cooler and I always WISHED I had a 1/4" drive, 16 mm, wobble socket for that= BUT no one makes one!!!!
Most guys, just pull a turbo and oil cooler, then the oil filter base, I believe. BUT if you try my way, just remember, that on reassembly, you must snug bolts up and inward evenly, so that base get evenly to the block, but also gets pulled upward evenly towards the oil cooler, too.

One more time, and this may only apply to C12 but maybe C13 , too.
That is, of those 6 bolts, for the oil filter base, going into the engine block, I believe 4 are blind holes, BUT two are NOT, thus they need sealant on them or oil may seep out the threads. If you do it yourself, just take a small screwdriver and put in each hole(while disassembled), thus you will know if they are blind, dry hole OR open and need sealant.

(C13 rear crankshaft seal and adapter housing.jpg)

(C13 rear flywheel housing example.jpg)

(C13 oil filter base example.jpg)

C13 rear crankshaft seal and adapter housing.jpg (43KB - 1436 downloads)

C13 rear flywheel housing example.jpg (55KB - 1352 downloads)

C13 oil filter base example.jpg (37KB - 1674 downloads)

Yes, they are reuseable.

 Just clean them up some. We pull of a little of the blue silicone, if we can, then steam those reuseable gaskets off, blow them dry.
Now, you still need to have the underneath side of the block clean, then add silicone to the joints, where both the front cover and the rear cover "seams" are at, then reasonably quickly get the oil pan up and tightened.(less than 5-10 minutes)

Note: I tend to hold the rubber oil pan gasket onto the pan, with some really small wire.(We call it tag wire). I just tied it loosely in 4 corners, put the pan up, get some bolts started, then remove these thin little wires, before bolts are tightened.
If you have two people, putting up the pan, you won't need to do this.