You could refer following pic. it show what i wanna do in site.
so i wanna confirm it by you, i'm not sure it is correct or not, If I think right, it will be like the picture below.
and for Network traffiic level is not too much heavy, which mean i don't need high avaliability, so Please recommend the most basic ones model,
Most important thing is when the Primary Server has problem, it must be disconnected automatically from the network and the Backup Server takes over the function with the same settings.