Pretty much i’d suggest getting something with at least 8gb of ram, even if it’s only for searching the web etc.
i’m guessing it’s a laptop you want not a desktop pc.
Although alot these days most seem not to have a dvd / cd rom although one can buy a usb plug and play one pretty cheap if one is needed.
Then i guess it comes down to screen size.
I bought a cheap gaming destop pc when lockdown started back in march from argos on the argos card at 0% for 7 months and has been awesome and so glad i opted for the 8gb of ram as programs seem to be using more and more ram and after a while older pc’s just become so slow because of that..
I did a college course years ago on many aspects of computers and building them for ones needs was one of the courses. The other main thing is the processor, i find the cheapest/best option is not to get the newest one out but the one it is replacing. Be careful of older ones that seem really cheap as usually they got much older processors. may seem a good deal but will be obsolute sooner.. £400 should get you what you need for sure and do a good job for quite a few years.