If you use your 1070s properly and set the powerlimit low enough that you can run the fans at 60-65 they will last a very long time.
1080ti cards do have high hash rates for many algorithms, but they also use much more power than a 1070. 1070s are great if you manage them properly. You much better off spending the money you want to waste on watercooling equipment and buying more 1070s.
Water cooling rigs do not need flushing every week if you use proper coolant. I have watercooled CPUs that have been fine for 6 months, but like i said - the blocks for the GPUs, the pump, the radiator/s, reservoirs, pipes, fittings, etc are all VERY expensive. They are for pimping your rig when you want it to look cool, not for serious mining.
I was looking at this video card
http://www.gigabyte.us/Graphics-Card/GV-N108TAORUSX-WB-11GD#kf
it doesnt look cool it looks like a video card, I dont understand are water cooled rigs more for show? Im trying to put 3 water cooled video cards in a PC, leave it closed and in the corner and leave it in a little old ladies house while it mines... little old lady being my grandmother. I not trying to have a flashing case with neon lights and attract attention. I would still need to buy the pump, the radiator/s, reservoirs, pipes, fittings even after I bought that card???