I'll probably go for TrueNAS.
I suggest you read up about TrueNAS to see if it's suitable for your needs. Although it's a sophisticated storage system, it can take a while to puzzle out how set up your array, vdev and sharing options, if you're unfamiliar with the OS. Luckily there are numerous guides.
https://www.truenas.com/truenas-core/
https://www.wundertech.net/how-to-install-plex-on-truenas/
I run RAID-Z2 which means any two drives can fail and (in theory) your data remains intact. In practice, you may encounter problems if one of the remaining drives contains hidden problems during "resilvering".
With an 8 disk array in RAID-Z2, you end up with 6 disk capacity, i.e. with 8 x 14TB (112TB), you'll only get 6 x 14TB (84TB) for your files. You can also run RAID-Z1 (single drive redundancy) or RAID-Z3 (triple drive redundancy). 10 disks in RAID-Z2 is better, if you have the room in the case.
Please remember that RAID is not a backup. You need copies of all important files saved on a least two other systems or storage media, with one copy off-site if possible.
I wanted to buy an Intel so i wouldnt have to buy a GPU, is this one ok just for transcoding;
TrueNAS Core runs on virtually any AMD or Intel CPU going back to around 2010. You don't need a large SSD either. I have not run any form of transcoding in TrueNAS so I can't answer your question, but the more cores the merrier when I'm rendering video in Windows. Buy what you can afford.
For a basic TrueNAS Core installation you need 8GB RAM and a 16GB SSD.
https://www.truenas.com/docs/core/13.0/gettingstarted/corehardwareguide/
However, TrueNAS Core "loves" RAM and the minimum recommendation is 16GB. Even this might not be enough, when you start running "plugins" and "jails". If you fit 2 x 16GB (32GB), any spare memory will all be swallowed up by the cache in TrueNAS.
There's also the thorny question of ECC versus non-ECC RAM. The purists would argue you must use ECC RAM to reduce the chance of data corruption. I have two HP servers with 60GB and 64GB ECC RAM respectively, plus two desktop PCs with 16GB and 32GB non-ECC. All four systems are running TrueNAS Core.
This might be overkill for TrueNAS. As already mentioned, the minimum SSD spec is only 16GB. In the past I've booted TrueNAS from 16GB Kingston USB flash drives, but have moved over to 32GB mSATA drives in the HP servers. You'll have to research requirements for running Plex, etc, so a 2TB might be OK, but I suspect 500GB will be more than adequate.
I live in Italy so the cards this is the only card available here otherwise I have to try my luck on aliexpress:
That Amazon card is a bit pricey, especially if it's a clone and not a genuine LSI card. Do you have eBay in Italy? I might be inclined to try AliExpress. I've been using them to buy things for the last three months and delivery is usually 7 to 10 days, plus they're cheap. I'm not sure about the Italian postal network with overses deliveries, but it might be worth the risk.
Last thing since the HDD are not SAS are they compatible with the LSI HBA?
Yes, you can run SATA drives on LSI SAS HBA cards. If you do buy SAS drives instead of SATA, you'll need different forward breakout cables. The power and data ports on a SAS drive are "bridged" by a lump of solid plastic. You can plug a SAS dual power/data cable into a SATA drive, but you cannot plug SATA power/data cables in SAS drives. See below.
I should point out that TrueNAS, unraid and similar OS may not be the best solution for your needs, but with a working LSI IT-mode HBA, you can play around with multiple drives and different software. It's all good fun.
I can upgrade the PSU to 700W but ill run a test to see how much does this setup consume.
Budget on around 10W per hard drive, but remember the motor startup currents can be high, e.g up to 2A each on the +12V rail. Eight drives powering on at the same time could potentially pull 16A at 12V for a short time. Professional multi-disk servers spin up the drives consecutively with a short time delay between each drive. I've not had any power problems with 10 hard disks in a desktop system. My HP servers have dual redundant PSUs and are designed to run up to 24 x 2.5" hard disks.
I'll start with 1 M2 SSD to install the OS and 3 HDD.
You cannot add more disks to a TrueNAS array and retain the data. It's wipe and start again when you add drives. No great problem if you're experimenting, but a pain if the array contains TB of data.
I check all drives with a lengthy Hard Disk Sentinel surface scan in Windows before installation in a TrueNAS array. On an 8TB drive this takes around 10 hours for a simple read scan which checks every sector thoroughly.
https://www.hdsentinel.com/help/en/61_surfacetest.html
With 14TB drives, you're looking at roughly 20 hours per drive for a read test and 40 hours for a write then read test.. Some people "scrub" their disks with long SMART scans for a week, before building an array. It's up to you, but with second hand (ex-server pulls) I like to check thoroughly for disk errors. It's not worth risking important data on untested drives, even if you have other backups.
If you see this result, use the drive as a door stop.