I wouldn't consider Bon Scott or Brian Johnson's singing gutteral. Screeching maybe, but not gutteral. Gutteral is much lower and IMO you can't even come close to understanding what they are singing. Think Slayer/ Slipknot/ Pantera and they arn't even the worst.
I like Rush and always have, but as I have gotten older I really enjoy them even more. Limelight is good, but never been one of my favorites. I like some of there synth stuff as you put it. Subdivisions is one of my favorite Rush songs. I like their instrumental stuff. 2112 etc. The documentary that was done about them was very good. They guy that did it also did a doc about heavy metal and its origins and genres. The guy is a big head banger with a PhD in anthropology, can't remember his name.