In my experience running a leak test with just a PSU does very little. It doesn't take in account the extra heat the system faces under load. The heat can cause leaks that you can't test because it softens the tubing to form new leaks.
Anyway, I recommend clamps for the fittings. They are just as good as compression fittings but much cheaper. Unless you actually have money to spare for compression fittings. It doesn't matter, clamps are a bajillion times more secure than not using clamp. It is an insurance policy that is worthy of your investment because most people can't afford to get their computer fried when clamps are so cheap. There are even fancy Koolance ones.
A radiator is still cooled by fans and the noise really depends on what kind of fans you put on them and the RPM. I recommend Gentle Typhoon, they are the preferred fans for watercooling due to their low noise and high pressure. You can get a fan controller to reduce fan speed.
Non-conductive fluid exists but they don't work. Any fluid will pick up anything and everything at an atomic level. Since the waterblocks are metal, some of that metal will dissolve into whatever fluid in the form of ions+ and start conducting electricity relatively quickly. After a few days, the most non-conductive fluid will conduct enough electricity to fry your components like a McDolands deep fryer.
In my experience, never use pre-mixed liquids no matter how awesome they may look under UV light through clear tubing. I recommend UV reactive tubing instead. UV reactive tubing looks amazing although to a slightly different effect. On the plus side, UV reactive tubing stays UV reactive for an extremely long time while UV reactive fluids tend to degrade and lose their light after a while.
The main drawback which makes me not use pre-mixed coolants is that it stain and leave residue. Im a messy person and I usually get my coolant everywhere, I don't want to double rinse everything when I spill the tiniest drop of coolant. It can also stain tubing, the res and blocks. I had such a terrible experience I feel the need to tell everyone about it. Manufacturers won't tell you anything because they are trying to make money, otherwise you would use straight water and that is really really cheap but awesome.
I use straight deionized water which can be purchased in an auto supply store for like $2-3 a gallon. They are primarily used for car radiators or car batteries because the water is extremely pure and won't cause mineral build up or corrosion which is what you are looking for. Its also easy to clean up and handle.
For radiators there are ones that take 120mm fans or 140mm fans. 140mm fans are quieter but the downside is that 140mm is not standard so you might have trouble mounting it. If you can't mount it then I would just go with 120mm, its not that big of a difference. You should go for a 360 (3x 120mm fan) radiator, there are double and triple thick variants as well if you want more heat dissipation.
Decent price is really hard. I can justify getting a nice CPU block because CPU blocks have universal mountings but GPU blocks are dedicated. Most GPU blocks are around $100 and for that price you can start looking at a new GPU. Unfortunately GPUs tend to be much louder than CPUs so I don't know what you want to do.