Do you want USB just because it would be cool to have (which is a totally valid reason, of course)? If not, maybe you should weigh the pros and cons - USB solutions for Amiga are expensive and hard to find.
Unless your A1200 is supposed to be used for modern desktop computing tasks only, USB input devices are a no-go since they don't work with Amiga games. Having two mice connected to an A1200 is just weird, IMHO. And using a 600dpi mouse with a 640x256 display doesn't sound like a perfect fit to me.
Nobody's using scanners or tablets on an A1200 anymore, that leaves USB storage devices. A 5 € PCMCIA network card and the free AmiTCP 3 from Aminet will let you connect your A1200 to your local network. Much cheaper and more comfortable than transferring files using USB sticks...
