You are correct in choosing a 5' cutter for your 8N.
Whether you should buy a light or medium duty depends mostly upon what you plan to cut and how long you expect the cutter to last.
Most light duty 5' cutters have a 40 HP gearbox, shear pin protection and a tailwheel equipped with bushings, which will be a routine maintenance item. Such cutters will weigh 400-500 lbs and are perfectly adequate for mowing grass, weeds, small saplings and brush that is not too heavy.
Medium duty cutters will likely weigh 500-600 lbs, may have a heavier gearbox (or may not), slip clutch protection (unlikely) and a roller bearing equipped tail wheel, which is an improvement well worth the added expense. You can expect such cutters to last longer than the light duty types.
In any event, do not buy a "true" heavy duty cutter as it will be too heavy for proper use with your 8N.