Hello,
Wattage just need to be enough to ‘feed’ the system… with a safety margin.
Corsair are pretty good, other specialised PSU vendors can be slightly better, although more expensive, like e.g. Seasonic.
I have 3 systems with Corsair (600W) and one with Seasonic (650W), they all behave very good and reliably. Note that these are all ‘Bronze’ series, decent efficiency.
But of course it depends on the configuration - Corsair and Cooler Master have a wattage calculator online tool: https://www.corsair.com/ww/it/psufinder and http://www.coolermaster.com/power-supply-calculator/
Which kind of use? If you are using big sample libraries, then yes, the more RAM the better.
The slots’ configuration also depends on the memory channels the CPU you are going to choose has - your nephew will know how to check 
If it’s negligible, by all means go with the bigger one.
But if you plan on adding other drives (say, one for libraries and one for projects, or a big one with two partitions), the space you’ll need on the OS drive won’t be much.
I personally use a 480 SSD for dual booting two Win partitions (OS + Cubase, plug-ins, stand-alone guitar and synth software and a couple photo programs): all libraries and projects reside on 2TB HDDs, the SSD is still half empty!
Depends on the usage, again If you need low latency, then fewer, faster cores are better.
If you work on big projects but don’t need to go low on latency, it might be worth buying into a CPU with more cores, but I would definitely recommend to avoid CPUs with many, slow cores (e.g. CPUs with 24, 36 cores running at 2.50).
I have an i5-4690K @3.50GHz, runs projects with 150 insert plug-ins and 4-5 (max 8-10) VSTi at 50%/70% (with buffers between 96 and 256).
A benchmark reference for newer CPUs (including high-count multi-core): http://www.scanproaudio.info/wp-content/uploads/2017/10/Dawbench-DSP-8700k.jpg
Good luck with the build!
P.S. The newer (7th and 8th Gen.) equivalents to the K-series i5 are the i5-7600K and i5-8700K (quad and exa, respectively).
On the i7 side, you have the 7700K and 8700K
P.P.S. Might be a good idea to hear other Komplete 11 users that have similar needs, if you heavily rely on it.