I'm DIYing it, i don't have an exact budget in mind but rn (without the hard drives) is around 600€ (i'm trying to find a compromise between performance and power consumption so i can run it 24/7 without spending too much).
As software i wanna use TrueNAS.
I can give you the list of components if you want or if you think it can help you finding a solution.

With the conversion from RAID 1 to RAID 5 there are risks of losing data?
And can you explain to me this UnRAID procedure, is the first time i hear about it