The first photo shows a coaming made of carbon fibre and epoxy. I've had great success making similar coamings but using fibreglass cloth instead of carbon. Both will work fine. The fibreglass cloth may be easier to find and certainly more expensive.

I've done a blog entry on how to make such a coaming. Here's the link:

http://justyakkin2.blogspot.ca/2012/11/point-bennett-project-lots-started-but.html

A spray skirt does a wonderful job of filling that big hole in you boat - the cockpit. Whether you need one really depends on what kind of paddling you plan on doing. In still water, you can go without. But paddling in any waves without a skirt is asking for trouble. You run the risk of waves entering your boat and potentially swamping you or, at least, the stability of your craft will be affected by the free surface effect of the water sloshing about in your hull.

A spray skirt is a must if you plan and rolling or even putting your boat on a good edge - the latter of which can dip the edge of your cockpit below the water. This is most true for boats with low volume.

I think you will find that most designers leave a gap of around 4" between the back of the seat and the back of the coaming but I stand to be corrected on this!! A foam piller or back band will provide support - which is must better than leaning right back onto the coaming.

You can easily make a seat out of closed cell foam. Here;s how I did mine for Black Pearl. This entry also show how I finished off another cockpit coaming.

http://justyakkin2.blogspot.ca/2012/05/black-pearl-project-punch-list.html

Hope this helps,

Sean