Need help selecting best options for new array

Greetings, I am new in this room and don't have Unraid experience. But since my server is not built yet, there is an opportunity to start off on the best foot. Also, even though it might be like 4-letter words here, I could consider straight raid 1 or TrueNAS if anyone thinks those fit better, but from my own reading think unraid will fit just fine - with a couple questions.

Planned use is personal and very small business use for backups, media server, file transfer, and then as a database server with either plain file and mySQL DB. It will only be me accessing the DB so high performance isn't an issue. Over a few years the DB and overall storage could grow to several TB, potentially over 30TB.

My biggest questions are (1) should I configure the components for ECC memory or non-ECC? and (2) since initially huge storage isn't needed should I look for more smaller drives then upgrade over time or start with a lower number of bigger drives.

Some price range info is a little further below and, importantly, price will be driving my decisions. I can give up some performance for cost but don't want to go too far.

On ECC: Should I pick ECC or non-ECC?

From both reading and looking for parts, the AMD Ryzen series has limited CPUs to support ECC, in my price range, which is trying to limit the CPU cost to under $200 at the beginning - I can upgrade the CPU later if needed. This means I could go with a Ryzen 5 Pro 4650G (supports ECC) or a Ryzen 7 5700G (non-ECC). Memory price between ECC and non-ECC is near the same - I found both ECC and non-ECC DDR4 at almost the same price so the decision really is the CPU.

On HD's: How should I begin the selection of HD's?

In the first few weeks there will probably only be around 5TB or less used. Over time, say 2 years maybe, there might be 10TB to 15TB - I have some use plans but don't really know for sure, it could be more or less.

I am trying to not overspend at the start (probably obvious since most people aren't looking to waste money...) but am not sure how to get the full picture on this due to all the tradeoffs.

There are server class manufacturer-recertified drives for great prices - like 12TB seagate exos for $110 or 14TB WD Ultrastar for $135.  Then on the smaller side there are new 4TB seagate ironwolf drives for $75

From what I've read the Exos, Ultrastar and maybe Ironwolf could have a lifespan, or MTBF, from 5yrs to even over 10yrs.

Because of the long MTBF I'm thinking that getting the bigger recertified drives is a better choice since the cost per TB is so low. A lot of the space might be unused for a long time though - so I am not sure that bigger early on is better.

Another question on how data gets written to the data drives.  Is Unraid spread data across the data drives or does it fill the drives one at a time?  Like, if I have 3 12TB data drives and have stored 4TB data is it all on one drive or are different files on different drives?

Depending on how data is stored, should I get say 5 12TB drives but only start off with 2 or 3 with one as parity and the other as data then hold the other drives on the side as spares and/or data drive upgrades?

Sorry for the long post and lots of questions and twists and turns in my question-post. Thanks in advance for any and all opinions, advice and information - all is greatly appreciated!