It is a bit of a shame to gut a perfectly good working A600 Amiga to fit a low powered PC inside. I would be very sad to see that happening.

However, what you want to do is perfectly possible, and you could sell the working A600 motherboard and floppy drive on ebay or amibay.com easily and they are both very sort after. Worth about £15-30 for the motherboard and about £10 for the floppy drive.

As for tacking your project idea. For the keyboard, yes there is a solution. It is a hardware device called Keyrah Amiga and can be purchased for £29.99 from Amigakit.com here. And it infact offers a very nice solution because in addition to interfacing the keyboard to a USB port, it also adds 2 digital 9 pin Amiga/Atari joystick ports that you can plug any Amiga joysticks into for use in emulators.

I've actually been thinking of a similar project using an A1200 case and keyboard. Fitting a mini-ITX motherboard inside the case. You can get motherboards that small that take Core2Duo chips, so you can get a fast PC out of it too.

To show you an idea of what is possible have a look at some of the projects over at mini-itx.com, and especially ones like the C64 one here.