I looked at the DIY skimmers, I tried building one a couple years ago. I was never satisfied with its performance. There is a fine balance to be achieved. The skimmer built from a utility sink looked promising.
I bought this skimmer for my pond.
http://www.petmountain.com/product/pond-filters/522567/proline-pro-5000-pond-skimmer.html
Regarding the drawing, the layout looks OK if the pump is inside a skimmer. The flow in a basic rectangle pond should be from on end to the other end. The waterfall weir will help keep the water moving and debris moving toward the other end of pond, skimmer will pick up large debris and floating trash, leaves are the worst. 3 or 4 large leaves can clog a pump intake fast. I am not familiar with an Oase 4000 filter but know that they are expensive. Why not use a large skippy style as your waterfall filter and possibly an upflow 55G drum prefilter behind the waterfall. You can hide it with rock and landscaping.