The IS200 is not the best car if fuel economy is the key factor, the manual is better than the auto but my auto averages around 24mpg, and if mainly town driving it's usually around 20-21mpg.
As above if you're used to an auto then the IS300 might make sense as there isn't a lot in it fuel wise but the car will feel like less of a power drop from the AMG monster you're currently used to (please post pics as I love AMG mercs!)
All the power is at the top end (kicks in at about 4k revs) so you do need to rev it hard to get the best out of it. The supercharger helps sort this out but that's extra expense plus they're pretty hard to get hold of nowadays.
In terms of fuel I'd think a BMW would probably make a better option, but I love my IS, it's a great car, still looks great, drives great and has been pretty much trouble free for the 8 years I've owned it. If you drive one and like it, and don't mind the not great fuel economy, then you definitely won't be disappointed.