if a bought a TFT with both digital and analogue imputs. Would I be able to plug the Amiga into the VGA and the PC into the digital and swap between them from the monitors front panel?
I think most allow this but it would be wise to check the exact spec of the monitor in question.
I've always found a dual input monitor very handy so when my trusty Iiyama Vision Master Pro (with DB15 and BNC inputs) died I got a Sony X17 LCD which has dual DB15 inputs and a single DVI input which can be swapped with a switch on the front panel - it's great. The X17 also has built-in speakers with two inputs and it can choose the sound input associated with the graphics input you're using.
Robert