So I just bought a Dell XPS 8900 i7 6700 from the outlet. I intend to upgrade to a a newer z170m mobo that will support non-k oc, memory upgrade to faster memory that will oc, bigger psu, and gtx 980ti, and clone old drive to ssd drive. What issues will I have with drivers, licensing of windows 10, etc.. I'm going to use a water cooler for the cpu.
Sorry, what you're suggesting makes no sense.
The Windows license is LOCKED to the Dell motherboard. You can NOT use a Dell OEM license on a non Dell motherboard.
The XPS 8900 case does not support a water cooler. Your only option would be to put the radiator through the front, and then you'd really need an air barrier to separate the exhaust from the air intake area above.
You should also upgrade the power supply. You'll also want to change the DRAM.
So, you're taking about another motherboard, another case, water cooling, and another Windows License. You can get a Windows 10 license for under $100.
So, by buying the XPS 8900, you got a CPU. Imho, buying an i7-6700K is better. They are specially screen to support higher clock rates. In fact, you'll be lucky if a non-K can clock as high as the base clock of the 6700K. Intel has been trying to support the K demand. So, people can forget about "being lucky" and getting a non-K that can clock as high a a K-version. The 6700K cpus are coming down in price.
Fwiw, I strongly suggest getting an ASUS motherboard.
Me: Geek to the nth degree. 
To build a Core i7 6700K from scratch with the parts I want would be at least $2000.00. But after getting the XPS 8900 for only $500.00, and pulling and selling the existing memory, psu, video card, mobo, I should be able to build this to run at a little over 4ghz(even up to 4.5ghz) ocing memory to 3000mhz all for around $1500.00. For a $500.00 price difference, I can not sweat that the K model will OC a few milliseconds faster on some tests.
I7 6700/16gb ddr4 2666 (over clockable)/gtx 980 ti/cd/dvd burner/1tb hdd/256gb ssd/850w PSU/mobo that supports oc or bclk oc/case/cooling
I know this is an old tread, but An XPS 8900 will work with 2666mhz ram. Kingston hyperX Fury will auto overclok on this board just like it does on any other z170 board. Asetek also makes a water cooler that will work on this board and case too. 92mm setup.