If you have a desktop and are happy with monitor, keyboard and mouse, my preference is these machines.
https://store.minisforum.com/collection ... 6661140641
There are quite a few, but that is a complete replacement with 0.5TB ssd, 16GB RAM, integrated graphics and Type C USB. You can add a laptop 2.5" hdd, either rotating or SSD quite simply by undoing the 4 screws on the bottom, clipping in the SATA extension cable and screwing the drive into the lid you just took off.
I have a MX400 from Amazon and a HX90 which I bought direct, barebones and fitted my own RAM and drives. If you buy direct there are some delivery charges but you can check the box and you get a bill from DHL. Mine was about €25.
Prices on Amazon UK are quite a bit more expensive. Possibly due to the windows install?? I never bother about that as I always use my own, but it doesn't look like the systems from the manufacturer come with Windows.
If you want a comparison of the CPU grunt, I chose the fastest processor from the Core 2 Quad range (2007 introduction) and compared it with this budget chip in the new box.
https://www.cpubenchmark.net/compare/AM ... 3881vs1033
As you can see, it outperforms those processors nearly 3:1 and that intel chip was way over $1,000 when it was new. I doubt your current machine has a processor anywhere near that fast unless it got an upgrade at some time.
The differences in disk, memory and graphics are likely to be even more dramatic.
Just some options. As WM says, options with Laptops are also very wide. You can also go to ebay and buy a barebones system (as my father did), but you'd need to buy the drives, OS, etc and it would come out to the same in the end or more.