Not sure if this is the kind of thing you're looking for, but as far as guitar pedals go, I think that the SansAmp 'British' pedal does a pretty good job with the AC/DC sound, and various Marshall sounds in general. Considering how much "classic rock" is Marshall-based, I think it would go a long way towards what you want. That being said, not all classic rock is Marshall-based, so you'll still have the Vox and Fender sounds to replicate. I think that the SansAmp 'Liverpool' and 'Blonde' pedals also do a good job in those areas, but admittedly then you're going to be spending a lot getting all of those pedals. I actually get pretty decent approximations from my Behri V-amp2, but then my needs are currently fairly small, as I'm just hacking around.