I have repaired four front seats on a 240D, 2 300D's and on a '82 300SD so I am pretty sure your car is similar.
There are three of the four you listed: Seat covering (leather) the seat cushion and the seat frame which has the metal spring assembly.
I have done combos of these. I have just replaced the cushion and not the frame, the frame and cushion etc. I am only discussing the seat bottom BTW, not the seat back.
You will get good results with just the cushion replacement, very good results with replacing the frame and cushion, but best results replacing all three. The problem is the seat covering can be VERY expensive as a factory part. If the seat covering is basically good, that is no tears or rips and the underlying padding/foam then consider frame and cushion.
The frame/spring unit as far as I know, cannot be purchased separately. This unit really makes all the difference in height and position. I look at the passenger seat and compare the two. If the driver's seat is definitely lower, especially on the front edge (with both seats adjusted to same location for the comparison) the do the seat frame.
This may be helpful: I used the car's mileage as a guide. If less than 100k miles then the seat frame might be OK. - If the frame has a break anywhere on its springs, replace it.
When I replaced the frame and cushion the results were always great.
As far as the repair- There are numerous threads on this site regarding the process. The seat is held in by four bolts, two for the front of the seat tracks and two on the rear. Don't disconnect the power seat connections until you've done these bolts as you need to move it front and back.
Although one strong person with a strong back can remove the seat, it is always better to have a helper to avoid cutting the interior as it comes out.
The seat bottom has a "ridge" that runs all around its edge. The seat covering is folded and is inserted into this ridge. The best way I found to release the covering from the edge is to have the seat site on something, like box, and to slowly push down on the edge of the seat to compress it. I have found that starting at the rear works best.
Once the cover is released you will notice that the cover is attached to the cushion. This assembly should slide forward off the spring assembly, If not, turn the seat on its side and look for the metal loops that that hold it on. These can be removed using a good pair of needle-nose pliers. They are sharp at both ends. Save these as the will need to be used again.
Now you should be able to start separating the cushion from the seat covering.
Hope this helps.