Question about Intel Nuc Canyon Nuc5i7ryh

RAM and HDD have to be bought separately.

I'm not sure because I can't test it, but the i7 should be able to do 4K. But it depends on what you want to do with it.

Mine runs Win10 flawlessly.

RAM, SSD or HDD must be obtained separately. It has 2 slots for SSDs, 1x M.2 and 1x 2.5". For example, a 500GB M.2 for the system and a 2 TB HDD as large storage is possible. Only one slot can also be occupied.

RAM, I have the following:
16GB: Kingston HyperX Impact (2x, 8GB, DDR3L-1600 (PC3-12800), SODIMM 204).

M.2 SSD, the following should work:
Samsung 850 EVO (500GB, M.2 2280)
A faster M.2 is probably useless, as it is limited by SATA 6Gb/s.

2.5" SSD/HD
E.g. Samsung 850 EVO Basic (500GB, 2.5")
or any 2.5" HDD.

It has two video connections 1x Mini HDMI and 1x Mini Display Port.
Mini HDMI brings up to 1080P
Mini DP has 4k support, 3840 x 2160 @ 60 Hz, at 30bpp, This according to specification. I can't try 4k myself, I don't have such a monitor.

Because of term graphics card, it is integrated graphics.

Win10 runs flawlessly on mine. With BIOS update to the latest version.

There are several energy options possible. Depending on them, the system feels more or less fast. With the energy-saving option, it tends to be a little slower. Maybe optimise here.
About 4k videos: I can well imagine that it can get tight there. The audible fan also starts at higher performance. You can set options in the BIOS to set the fan to quiet. But then the performance will probably be limited if the processor gets too warm.

I have 2 monitors connected:
(1) mini DP to 4k (3840 x 2160 @ 60 Hz - with DP Mini > DP adapter).
(2) mini HDMI to 3k (2560 x 1440 - with miniHDMI > HDMI adapter)

OS: Windows 10
Works flawlessly.

I have connected the NUC to a 4k TV and it runs without problems in 4k/30hz with HDMI. Unfortunately, Displayport to HDMI 2.0 adapters are not yet available, so 4k/60hz is not possible at the moment.