Maybe this looks like a start WTS Synology DS218j + WD RED 4TB (CMR unit) or can look into DS220+ (2GB RAM) x2 2TB IronWolf? 

That is too small budget and size. Save more.

Just movie alone is killing your storage size. You not keeping 480p movie right.

Can. Custom build NAS. Use FreeNAS or other OS.

Pentium G6400 - 239
Gigabyte H470 HD3 - 599
8GB DDR4-3200 - 159
Cooler Master MWE Bronze V2 550W - 239
Fractal Design Define 7 (up to 14 HDD) case - 699

Nope. Its not hard to assemble PC anyway. Got plenty of videos for it.

Pros:
A lot cheaper.
You can do whatever you want. Like hosting the website yourself, or setup game server.
CPU and RAM upgradeability
M.2 PCIE or SATA support
Amount of HDD you can install.
Can transform it into regular PC.
Expansion like SATA cards, Thunderbolt, Video capture cards.

Cons:
Hardware assembly required
Setting up OS takes time. Usually only one time setup.
Consumes a bit more power. (if you think 20ish watts idle from the wall on Pentium 6400 is too much)
Casing can get very big

if you haven't started the journey of building your NAS, you should seriously consider Synology being a strong advocate for them because of its reliability and ease of use OS. If you are technically incline, suggest you get the plus models ie DS220+/DS420+/DS720...... and the list go on, coz it x86 architecture good for dockers and more. Nowadays we all have "good" internet most of the time anyway if you are in the city area. A simple NAS with mounted cloud drive i.e google/dropbox/box will give access to more space than you will ever need especially you are wiling to shell out for a single user google enterprise account for now is still unlimited after the change to google workspace, business accounts has be capped and probably lose the unlimited storage sometime down the road. look up rclone to do file transfers. some chinese group released gclone some time early last year which bypasses the 750gb daily limit. unfortunately, it now late in the day and google has capped it down to something like 2TB daily. many have been moving hundred of TBs for the longest time but like all good things must come to an end

is a fact synology OS is just a very polished *nix GUI. Looks pretty and easy to setup, they follow the Apple philosophy, keep it simple, less is more strategy. well try out their demo at demo.synology.com you will see how fantastic their OS is. I am in no way affiliated to them but have started using Synology since day 1 and have been very fond of their products and services.