A towed disc will give slightly better ground penetration because all the implement weight is on the discs, none is supported on the tractor three point hitch. However, it takes a lot of room to turn a towed disc 180 degrees and you may need transport wheels to move a towed disc over the road from field to field. Turning and transport are the reasons three point hitch mounted disc harrows outsell towed disc harrows by a wide margin.
To a certain extent you can adjust draft force by adjusting gang angles. More angle, more resistance. Less angle, less resistance.
You probably want a Tandem Disc Harrow not an Offset Disc Harrow.
You should be fine with R4's so long as the field is not muddy.
If you want to chop and bury crop residue get notched discs at least on the front section.
If your soil is rocky you will want thicker discs, especially if you decide on notched discs.
You will probably want 20" diameter "wheels" but possibly 22" diameter, depending on how deep you want to till and how much total harrow weight you want on each wheel.
You will likely want nine inch spacing between wheels, not seven inch spacing between wheels. Fewer wheels, less float, more penetration.
You need to be able to pull your disc at 4 - 5 mph for max soil mixing.
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