If you are buying, pump is a great place to start and I would only recommend a D5 vario (suggested by 4ryan6 above) or a MCP35x.
Laing D5's (commonly known in the US as MCP655 or PMP450) are very high flowing pumps and quite powerful, yet silent. Variable speed is useful as not everyone wants to run theirs at 5, and you may not need to, either. There are 'B' versions which is not variable speed; it is permanently set to about 3.5-4 on the scale of 1-5 of the Vario.
MCP35x is a Laing DDC pump (Swiftech is the only maker of the MCP35x to my knowledge) as it runs a special PCB with PWM support. This means you can use your CPU_FAN header to power the pump and utilize PWM with this pump to speed up or down based on CPU temp. You can also utilize it at 100% or with a controller if you wanted to go that route as well. DDC pumps are commonly known in the US markets as MCP350, MCP355 and MCP35x. All use the same pump size, similar rotors, similar PCB and power (exception being 35x) and all can utilize the same pump accessories/tops/mounts as they utilize the same thread patterns for top attachment.
Before a radiator is suggested to you you need to disclose your overclocking intentions of either the 2600K or 2500K you own, if you intend to stay around the 4500mhz range a GX240 is as comparable to a Noctua NH-D14 air cooler
EX or RX? Either would be a good choice over an RS model, if sticking with XSPC. Of course, there are other radiator manufacturers out there as well.
if you intend overclocking past 4500mhz then you'll probably need a quad radiator just for cooling the overclocked CPU.
You could probably get away with a 360 rad worth of space if you were low on real estate. Even a quality thick 240 with great fans could suffice, but you'd be pushing it. However, K-series chips that are clocked 4.5+ put out a lot of heat and need to remain cool, especially if you are running more than 1.5v to keep your CPU stable.
If OP is waiting on GPU's at the moment, I would recommend a decision on what overclocking goals and future plans are for GPUs before really suggesting anything. I know we've gone through this 'what-if' set of questions a few times now, so I'd like to nail down what we're actually looking at in order to match up what he's really going to need.
Either way, one of the XSPC kits with the D5 pump upgrade would be a decent buy, as would a Swiftech Edge/Ultima kit outside of suggesting individual components one at a time.