Those specs aren't suggested, they're required.
You MUST have 8GB RAM to run FreeNAS. If you want space to run jails, have any appreciable ARC, etc. you MUST have more.
If you value data protection (and we assume you do if you're going to the effort of building a FreeNAS), ECC is HIGHLY recommended.
You SHOULD have an HBA that's well supported. The Highpoint crap isn't. It's crap, always has been. The drivers aren't well written, and many of their devices really don't have a way to turn RAID off. Can you make it work, sure. When you come crying, saying your pool has been lost? The first comment will be "we told you that RAID card was a bad idea".
There are thousands of people and many thousands of builds contained within these forums. The recommendations exists because they work, and work well. If a guide says "don't do X", it's probably not just someone's opinion. It's because someone has been down that road and learned the hard way.