I've checked a workstation from ATL to MAN a few times. Delta will generally make you sign a waiver whereby you assume all the risk, but no extra charges provided it is within the normal size/weight. I recall paying $50 extra because mine was 60lbs - but that was a few years ago. I had mine packed in the original Dell box with extra foam.

I removed the HD's and packed those in my carry-on. I think you'd be crazy not to do that. That shouldn't be an issue for customs since you'll have to clear with all your luggage.