It’s often better to buy a refurbished Dell, HP or Lenovo PC from the original manufacturer. They are best placed to refurbish them, having a supply of second hand parts. (You can’t guarantee that third-party refurbishers will use original parts.)
Dell sells refurbished PCs at dellrefurbished.co.uk and dellrefurbished.com. HP sells them at its Business Outlet online store in the USA, and via resellers on its HP Renew programme in Europe. Systemactive is a UK example.
Lenovo has an outlet store in the USA, while Lenovo UK recommends buying new not second hand. However, it does sell “certified refurbished” PCs on Amazon.co.uk.
For UK buyers, Tier1online is a good source of ex-corporate PCs, and will guarantee some of them for up to three years for a small fee. (Tier1 is an asset-management company, and Tier1online is just a sideline.) Many other UK retailers stock refurbished machines, including (at random) Encore, Microdream, Laptop Outlet and Laptops Direct. Amazon and eBay are also potential sources.