Thanks everyone
Following the general advice that the machine we were looking at wasn't ideal after a lot more research we finally found this one which seems to tick all the boxes so fingers crossed.
https://ao.com/product/90n900fduk-lenovo-ideacentre-g5-desktop-black-90976-253.aspx
Next task is to work out how to retrieve saved SIMS game data from the old hard drive to put on the new PC...
@Tolkienfan That one should work out quite well: you should be able to run all Sims 4 packs together on ultra graphics settings.
As for your game data, it's all stored in Documents > Electronic Arts > The Sims 4. You could move the entire folder to the new PC using an external drive or USB stick, or you could upload it to a free cloud storage service (Google Drive, Dropbox, etc.) and download it onto the new computer. You could also just transfer the saves folder and anything else you want to keep. In any case, I'd suggest zipping the folder(s) before transferring them—this helps protect some files from being altered and also saves space if you're using cloud storage.
The best practice is to launch the game once on the new computer to make sure everything works properly, then transfer the files to the appropriate subfolders within the Sims 4 folder. That's more reliable than transferring entire folders.
