I'd go for a refurbished Windows 10 machine from A C Computer Warehouse. (I've bought loads of stuff from them, both for myself and on behalf of friends. I've also recommended them countless times here on AB, with nothing but positive feedback from those who've gone on to buy from them. Their refurbished computers are like new, with a 12-month warranty, and their service is second to none).
I'm currently considering replacing my Windows 7 machine and this is what I'll probably go for. (I've also recommended it to a friend who's also moving on from Windows 7):
https://accomputerwarehouse.com/product/hp-prodesk-6300-intel-g870-3-1ghz-dual-core-8gb-ram-240gb-ssd-win-7-or-10/
I like the fact that it's go 8 GB of RAM and a solid state drive, both of which should see it really whizz along. However if you're simply looking the most economical model suitable for your needs, this is actually perfectly adequate for them:
https://accomputerwarehouse.com/product/hp-elite-8000-usdt-c2d-e8400-3ghz-4gb-ram-250gb-hd-win-7-or-win-10-pro/
(A C Computer Warehouse will get your computer to you within 1 to 2 days for a £10.95 delivery fee but if you're prepared to wait a little longer, for 3 to 5 days, delivery is free!)