A 460MHz 603e based processor should be able to emulate a 68040 pretty damn well, probably one at around 100MHz speeds even, maybe 200MHz depending on the quality of emulation. How is the state of AmigaOS 4's JIT emulator anyway? Petunia, that's the name.
As for the integrated Paula sound, I expect that it is an audio input header on the card that you connect to the A1200 motherboard's audio headers, or outputs.
If you want to do hardcore video editing, you'd be buying a Mac or PC anyway.
You'd attach the A1200 if you wanted to run software that required AGA. Personally, I'd keep the two things separate, new computer for new stuff, old computer for memorable stuff.
