Water cooling part selection is not the for the faint hearted. Takes hours of planning really.
Step 1) Complete parts list (and what you want to water cool)
Step 2) Choosing your radiators
Step 3) Choosing your pump/res
Step 4) Tubing size/type
Step 5) Fittings (2 per item in the loop, minimum, additional drain (valve) and fill ports are recommended)
Step 6) Water cooling blocks (This may decide Step 5 for you)
Step 7) Fans
Do a layout and see how long the runs are, order double the amount of tubing you need (hardline sucks when you make a mistake)
Plan your bends.
If you aren't bending, triple the fitting budget (or more) As you will need a lot of fittings to get around corners and line things up.
There are easier ways. EK sells a distribution plate/res/pump for the O11, though that will be a good chunk of your budget. That basically defines for you where everything will need to go except radiators.
Rules of thumb: minimum 120mm radiator for each device in the loop for adequate cooling equivalent to mid-range air coolers. More is desirable.
Thick radiators tend to have less fin density, so double the thickness is not double the performance, but still better than your average radiator.
$400 might not be enough for a complete system from scratch, depends on how high end you want to go.