Boss has a pedal that dials in both overdrive and distortion give that ago. I prefer boss' sound over digitech as digitech is more geared to metal heads in my opinion. As an overdrive pedal i like the ibanez tube screamer...one of the few ibanez products i like.
I love gnr, but i would dare try to play their music on single coils. Also i prefer tube sound for them, which your marshall might just have.
Try before you buy as well, that is much too important not to state.
Also remember to sound exactly close to slash is very hard, just try to get close, cuz there are too many variables.
Boss has something called a virtual pedal board that can help somewhat narrow done the tone you want at www.bossus.com
Digitech also has a interactive pedal menu and this is there overdrive pedal http://www.digitech.com/products/BadMonkey.htm , they also have a pedal http://www.digitech.com/products/tonedriver.htm and theres a distortion here http://www.digitech.com/products/hotrod.htm .
WWSD? What would stevie do?