A single 240mm rad would be sufficient enough for cooling to start with, even if you get both rads and run them as a single loop you will get really good temps.
The one thing I am worried about is the fan configuration. You shouldn't have a problem if the fans have a high enough cfm but by using such large fans you aren't really pushing the air through the rad fins, you are just swamping the radiator with air. Due to the rads not getting a strong direct air flow through the fins you may get worse temps.
As for the pump. The D5 is a very good pump, especially if you get the variable speed version. The DDC is more versatile mainly due to it's size and abundance of aftermarket tops offering various input/output configurations and reservoirs. I do think the D5 has a slightly higher flow rate so for a single loop it will cope slightly better then the DDC but the loop won't be that large anyway so either pump will be fine. As for mounting the pump, it's purely up to you. The pump will function in any orientation, you just need to make sure you do the plumbing right, each pump has a designated input and output, just keep that in mind.
If you did want to save a bit of cash get the DDC and get a res top for it, in most cases it will be a bit cheaper then a dedicated pump and res setup, plus it makes it easier when filling the loop (due to the fluid being added directly to the pump, there will be no dry running). The only downside of the pump/res combo is the capacity of the res but the pump will be able to push the fluid through the rads quickly enough to keep it quite cool.
Personally I would consider getting 120mm fans for the rads or make a shroud for the rads out of alu, that way you will get more direct air flow through the radiators without having to get new fans. As for the pump I would personally get a DDC with a res top, the versatility, size and relatively low noise rating wins every time for me.
Hope that helps .