Well if it's used then its entirely circumstantial, but if its new a build then a £400 + is pretty much all you need.
https://uk.pcpartpicker.com/list/FG3QwV - quick build
(Assuming you have the basic software and peripherals)
Here's a basic of basics build. It's probably not the *best* build you can get for the money, but it's definitely not bad. Would even allow you to play TW:WH at med-high settings at a reasonable fps of maybe 30-50, ofc, this fluctuates depending no the amount of units loaded on-screen. Would pretty much destroy Napoleon to oblivion.
I don't really know your budget, nor do i know which country you hail from, so that includes currency and such.
If you do, however, decide to follow this build, then my first...