If you're going to be a session player, or in a cover band, then the MFX unit is a good choice, as it will allow you emulate many different sounds quickly and easily on the fly. If not, you might be happier with the pedal approach. I'm like Razbo. I've owned & used MFX units, then right up to a rack unit, and then back to floor pedals again.

AC DC sound is easy. Just go direct into the amp, and get all the overdrive from the amp itself. Your neighbors are going to love you.

David Gilmour is tough. You'll want some delay, but his sound is not as overdriven as many people think. The tricky thing with him is that really, his distinct "tone" and sound comes from his playing technique, especially his right hand. I bet we could try out his full-on concert setup, and really not sound anything like him. But, doesn't mean you shouldn't try! So not too crunchy, and a little delay for the lead sounds, more if you're going for the more obvious delayed guitar style.

Putting an amp simulator in front of that amp would be absolute heresy by the way...