You will want at least a 2x120 per CPU or GPU component. You really wouldn't be buying a single 1x120 rad anymore anyway...most rads bought are dual or triple 120s. The Feser rad you have listed in your first post is a 2x120 or commonly called a 220...meaning you can install 2- 120mm fans. So, you can get 1 radiator, but it might be a 2 or 3 x 120 radiator...or the surface area of such.
Petra's Tech Shop is a good e-tailer. I admit, I have used FrozenCPU a lot over the years, but they are a little more than some of the others in terms of cost. However, they do have a much larger selection of just about everything if you need it.
Your fittings to your components...you can get either snap clamps or those little radiator clamps with the screw adjustment on them (what I use)...about anything similar on this page (minus the crappy one second to the bottom)
You don't have to get them there, I just knew there was a page to reference. I always get mine at the hardware store.
You will want to use distilled water ONLY as your base...you should add some PTNuke as well or some other anti-creepy-crawly growth inhibitor. You can add dyes and such if you want, but they rarely do much more than add color or UV reactive color.
Water leaks...this is why you leak test and make sure you have good connections. (another MAIN reason why I don't trust anything that comes pre-manufactured.) Take your blocks apart, clean them out; they often have bits of debris in them...same thing for your pump...at least flush water through them. Make sure you flush your radiators too...I usually fill mine about 1/2 way and slosh water around at least 4-5 times. You'd be surprised what comes out of them.
Make clean cuts on your tubing, don't 1/2 ass it. Clamp the tubing well, feel confident about the connection or don't move on.
Prime and test your loop...remove the ATX connector from your motherboard and jumper the green to any black wire to power your system. You will need to start/stop several times to get it going...then tilt/tip to eliminate air bubbles. Let your system run for several hours...12 at minimum...18 is better.
If there are no leaks, you are good. I suggest using a lot of paper towels on top of washcloths to allow you to see any drips (and to protect) your components...like video card, etc. If you do get water on anything, remove your video card, RAM, etc...use canned air to blow out excess water...use a house fan and let it dry for several hours. Hopefully you used distilled water and you lower the risk of shorting, but it's never a 100% safe bet with any additives or water.