Up to you. Custom offers performance advantages you pay dearly for in a prebuilt but, the prebuilt comes with a warranty and support.
For the basics you mentioned and nothing performance related like gaming or video editing it might be easier to pick something up that works out of the box. Budget options are out there and looking for 12th gen Intel would be ideal to last another 10 years and if they decide to add performance gear it will support just about anything today and through the next few years.
The 2.5 sata works fine then slap it into the new box or clone it to the new NVME in the new PC and use the 2.5 for additional storage as a 2nd drive.
I built my current "server" for $1500 but, tweaking things down a bit could bring that down to ~$1000
Of course once you dig into things or think of something else the price goes up but, it's going to offer performance based on needs not just the generic for the masses you get with retailers.