ECC is not a hard requirement for me, but if I can get it - I’ll try to, as to me it makes sense for something that runs 24/7 and handles my personal data.

I have a very strong aversion to separating storage from my server. I just don’t see why I need to route power and network to 2 small boxes (none of which would do what I need it to do on its own + considering very crappy room layouts in rented apartments) and then fiddle with network access, when 1 slightly bigger box would do what I need it to do. Some 7-8 years ago I’ve bought dirt cheap second-hand Huananzhi x79 with Xeon E5 and DDR3-ECC with some low profile NVIDIA GPU and it all still works now - and something like that would mostly be OK for me even now (except I left it in another country).

That said, it’s possible a reasonably powerful NAS will be enough for me on its own?

storage box and a services box

That’s exactly what I want to avoid though. I see no reason to power and network 2 different small boxes when just one slightly bigger one will do. And as mentioned - 500 is without HDDs, I plan to use the ones I have for now and extend it later.