I would skip the K1 and just go with a static media like lava rock or ceramic media, or something like that. It will give you a higher bio capacity for the space.
For mechanical, I definitely recommend filter socks. I have foam and filter floss in my sump, and I really wish I would have gone with filter socks. I might still switch over if I ever get the time to put something together.
For flow rate, I recommend you look to get at least 5-6X the capacity of the tank. So, for this tank you’re looking to get about 3-3500 gph, depending on if you want to turn over the capacity of the tank or the capacity of the tank and sump together. I’m a big fan of the Jebao DCP pumps. For your tank, I would recommend two if the Jebao DCP 15000 or 18000’s. The 18000 should handle that flow at about 50-60%, which is about 75 Watts each pump. The 15000 you would have to turn up a bit more, but should still be able to do it, I just don’t have a curve. With these pumps, I have had great results running my 8000’s at about 55-60%, so I recommend that others try to get around that as well. I have heard the pumps can make a high pitched whine noise if they are turned up higher. My 8000’s are completely silent at 60%.