The AmigaOne and its small successor the MicroA1C are difficult to find but to those who persevere they can be had. A good place to look for one would be the Amiga dealers on
AmigaWorld.net's front page. But if I were you I'd wait and see what comes along by way of accelerators for your A1200. You'll get more backward compatability that way. Otherwise you'll find yourself running most of the old software under EUAE on the AmigaOne/MicroA1C.
Rumor has it that the Petunia JIT compiler will work in conjunction with a future release of EUAE but in the meantime you'll probably be better off getting WinUAE for your Windows box in terms of compatability.