You may need more than x4 90 degree fittings, but it's impossible to know until you start building. I have 9 in my current build, but that's just how it ended up with wanting to run my tubes a certain way. You could feasibly do a build with none, but that would be trickier and take some careful planning (not to mention some genius bending lol!). I also find the 8mm extenders incredibly useful.
Are you cooling CPU + GPU? If so, I'd suggest more than one rad. It's do-able on one 360, but your temps won't be great, and nor will noise levels if you want them to be, as fans will need to work pretty hard. What case are you going with?
It's good to have more tubing than you need, it can be tricky to start with. Do you have correct sized silicon insert for the tube, and a heat gun?