You do not need to buy an All-in-One PC with a touch screen to experience Windows 8, but touch screens do make Windows 8 easier to use. You can use your old monitors with a new PC that has Windows 8 installed, just make sure that the new PC has outputs for two monitors. Windows 8 can be used with a mouse and keyboard as many users here will testify. I have no experience with All-in-One computers but a Google check would tell you if there are any models with the facility to add another monitor.
My personal feelings are that I like Windows 8 on my regular PC and monitor using a keyboard and mouse. It is fast to load and with a scroll mouse for vertical and horizontal scrolling I find it easy to use. I am not sure if I would like a touch screen with a desktop PC as I like to sit back in my chair and read the screen while using my keyboard and mouse. The idea of having to lean forward to reach the screen does not appeal to me. That said I would quite like a tablet with Windows 8.