I've had a watercooled computer for 15 years now. But, admittedly it's not for everyone. In that amount of time, I only had one small leak. The plastic reservoir cracked right where one of the barbs screwed in. I was warned this reservoir was prone to crack and after about 4 or 5 years, it did. I probably overtightened the barb too. It was a bay reservoir, so the component directly below the leak was a CD burner. So, I got lucky I guess, only cost me a cheap optical drive.
First you need to decide what diameter hose to use. I use little ole 3/8" ID that is 1/2" OD. Some like to use 1/2" ID hose. There are pros and cons to both sizes. I started out with 3/8" and never saw a reason to change.
There is no one size fits all recipe for custom building your own loop. I have a fairly simple loop, I can describe it in detail, maybe that'll help. Let's start at the reservoir, since I already mentioned it. This is just a place that allows easy filling of the loop. It helps a lot to get the air out of the system when you first start. A reservoir is handy to watch the coolant level. While I and many use a reservoir, it's kind of optional. You can install a T-line with a fill port (I've done that when I didn't have enough room for a reservoir). AIO units are sealed and I tried one like that before. They're convenient at first, but they don't last in the long run.
Ok, my system has a reservoir and 2 barbs and 2 clamps. You can plumb your system anyway, but to me it makes the most sense to use gravity and run a hose to the pump mounted lower in the case. The pump will have 2 barbs and 2 clamps. My loop goes to the CPU water block next. You have to make sure you get the right kit to mount the block onto whatever socket you plan to use. The water block has 2 barbs and 2 clamps. A piece of hose runs to the radiator (you'll want a minimum of two 120mm fans on the radiator, but if you have room, go for a triple). Guess what? You'll need 2 more barbs and 2 more clamps. Run a hose from the radiator to the reservoir to complete the loop. I know the next question, how much hose (flex tubing). If you decide to go the rigid tubing, you'll need someone else to help you. How much hose depends on how big your case is and how you route the hose. I try to route my hose in the most direct manner, so I probably have not much more than 4 ft. of hose. I would buy at least 5 ft., but urge a first timer to get 6 ft. (2 meters) to allow extra in case you cut a hose too short (yes, I've done that). There you go, simple, really.
