1| With the way things work with custom watercooling, you're first job is to educate yourself regarding the ins and outs of the world of watercooling. Narrowing down parts can be done over time but patience and understanding is key to this as you would while assembling a desktop system. You've also figured it our by now that you'll need to assemble the watercooling loop as well as maintaining it when the time comes. For that very reason its advised to read through the watercooling sticky{linked in my sig) to get a better understanding. Like a build, you have a lot of avenues to customize the loop thus our suggestions would be what and how we'd expect from a loop when you can and will have a different approach.
2| 340USD's is a good budget though with a 20USD increment you're not going to get much. The cost of a full cover waterblock for a reference 980Ti will set you back about ~150USD.
3| You'd also benefit from doing an image search for inspiration on how you'd like your loop to look like. Copy pasting isn't advised as this is your canvas and as all artists like to think they are in control of their expressions, well, you have it here as well