I think mostly the recommendations here doesnt seems to conclude anything lol.
Even if he were to buy a X34 monitor. At that resolution, you dont need a 1070 SLI? Mind that, 3440x1440 isnt far from 2560x1440.
Lets filter out what we think its recommendable. SLI and Xfire doesnt seems to benefit anything in any of the current gaming titles. Neither does it scale that well.
By adding additional card, it doesnt give you x2 performance. At most its gonna add 20-30fps range more. But with frame variance all over the place, it is not worth adding the 2nd card. So a dual SLI 1070/1080 is out.
Next, what does Z170 has that will benefit TS? Does he even planning to overclock? Neither does he use any of the features of Z170. Yet alone an unlock i7. Or even the next gen Kaby Lake.
When TS dont intend to overclock, there is no need for watercooling, neither does he need a large heatsink, a simple cooler will do other than Intel heatsink if he has aesthetics in mind.
In all i would recommend a normal B150 mobo with a lock skylake i7 6700. Attached it with a single GTX 1080. Together with a bigger SSD perhaps 1tb and do away with HDD (not unless TS installs a lot of things then i would recommend a 480gb/512gb ssd together with 2tb HDD). A good PSU 600w probably from Superflower, Seasonic X series. And add on with a good standard heatsink cooler, and lastly a pc case that TS desires that looks good.
All this should not add up to the cost of $2.5k