RPi5 is a good choice and the 4GB board is more than enough for LE use (2GB would be fine, but doesn't exist). As long as the NAS and RPi5 are both connected to a Gigabit Ethernet switch the connection should be fast enough.
Perfect. Thank you very much 
Do you want to play files from a library of test media or watch movies?
If you want a device that plays maximum test media, including HDR10+/DolbyVision/AV1 it's probably a new-ish Android or high-end Windows device that natively supports HDCP and more codecs than an RPi5. There are few devices that can do that under Linux and none of them are supported by this distro which focusses on the upstream Linux kernel not downstream vendor/BSP kernels.
If you want to watch movies: 99% of the 4K media files in my library are 8-bit/10-bit HEVC, and some are HDR/HDR10/HLG (or have compatible fall-back to those static HDR formats) and almost all are 23.976fps (a handful might be 30fps) and 100% of them play nicely on an RPi5.
Hello, I'm brand new here but I've been using Kodi on LibreElec for several years 
I have a Raspberry Pi 3b+ from 2016, and I'm limited to x264 encoded movies on my 1080p TV. I'd like to upgrade my setup at a low cost, especially to switch to x265/10bit movies, still using Kodi on LibreElec for simplicity, and it seems that the Raspberry Pi 5 is a suitable solution in my situation.
Just one detail: could you tell me if I should go for a 4 or 8 GB RAM version?
Another question, regarding the throughput of my NAS: it's important but I'm not knowledgeable about this subject at all, I don't know where to find the information. Could you tell me if my NAS HP Proliant microserver N54L will be sufficient in terms of data throughput, for now for 1080p, but perhaps in the future for 4K? My switch is a TP Link TL-SG105.
Thank you for your assistance.
