Old Style,
Sorry such a late reply, I'm a new member and your post caught my eye.
I have a 1500D (approx. 18hp, 1500lbs. and is 4WD). I moved recently and my new home has never had a garden. I just plowed it last week in my side yard. The ground was a little hard and I hit some clay areas from time to time. I have a 12" moldboard plow and use 3-45lb suitcase weights on the front. Sod is hard to plow in and of itself, besides the clay factor... my tractor had all it wanted. I had plenty of horsepower- it was traction that I suffered. I got the job done but had to be ready to feather my hitch up a little in the hard spots.
I would think a single 12" with your tractor weighted (depending on soil) wouldn't be problem. How much does your 1820D weigh?