Not sure what's available right now price-wise at 800 lbs, but I wouldn't necessarily make that the magic threshold for your tractor and the uses you mention. E.g., the Woods/Gannon BSM84 is 750 lbs, and very well made. Rated as "medium duty" by Woods, 40-65 hp, but I think they're being quite conservative. Only about $1400 new.
One dealer did quote a similar medium duty blade and wasn't really interested in quoting me the heavier one, so I figured that would work. But I think I will try for a heavier duty one for now. I can do more damage that way!