Your B and C should be pretty close together HP wise. The late B and the C used the same engine. The book I have shows the late B as having 19.5 drawbar HP and the C having 18.4 drawbar HP, both on gas.
As for the brush hog, a 5' unit will be about as big as you will be able to use, especially in heavy grass/weeds. Disc size will probably be about 6', maybe bigger, but I doubt it. I'm not familiar with a 19B disc, but if it's about 6' wide you should be able to use it ok.
The only real difference between the B and C besides the front end is the C is designed as a two row tractor vs. the B as a one row tractor. Just a difference in the width of cultivation. This does not mean the same thing as one PLOW or two PLOW, just the ability to cultivate two rows at once instead of just one.