unless you plan to build several pools/VDEVs, you are best to start out with the correct number of hard drives up front. You need to know the capacity you need for data storage for the next 5 years (most drives will last about 5 years), and then double that capacity if you are a storage hog. So you have capacity now. Next you need redundancy, how many drives can fail before you lose all your data? Many of us use RAIDZ2 and two drives can fail. Next, how many drives do you want to have in your system? Some folks are limited by case size or other factors. I'd recommend using a RAID-Z calculator (like one of my links below) to play around with the numbers. Maybe you only need 5TB and RAIDZ2, then you could see if it's 4 drives, 5 drives, or even 6 smaller drives. Maybe you want 4 drives so you can space them out and keep them cool (that is what I do now).
Pro's and Con's to think about: Smaller physical drives resilver faster. Many smaller drives crammed together get hotter. Larger drives are generally cheaper for capacity per dollar. To upgrade to a larger capacity you need to replace all the drives, it's not incremental, more drives could cost much more money. If you want to change the number of drives in your system, yo need to destroy your pool and rebuild it.
I really like TrueNAS (I started when is first came out as FreeNAS) and I have no complaints. You get what you put into it. It sounds like you are going down the right path, getting a good quality motherboard, putting in 32GB RAM (Question: is that a single stick or two sticks? I would go for two 16GB sticks so you run in Interleave mode, and if one should fail, you still have the other to run the system.) Some folks really love that 804 case but myself, it's too small. You are cramming a lot into it. If this is the path you go down, when you put it together just think Air Flow! Use zip ties to bundle the cables nicely, if you need to add more fans or better fans, do so (you may not know this until after it's together), and keep it in a cool place. Check those system and hard drive temps. If you are running near 50C, that is too hot in my opinion to continuously run. You can look in my description for my case and you will see a fair sized full case, lots of airflow, cool drives. And go figure, it's quiet too.
Mixing drives from different placed, manufacturers, lots, etc was a thing done more in the past. Drive manufacturers had a period of time that things were just not going well. These days most manufacturers are doing better, just stay away from Archive/SMR drives.
Best of luck to you.
