Hi Rob! I’m not sure offset disks are considered primary tillage since generally you would want to chisel or plough the soil before running the discs (or they wouldn’t really sink in all that much unless you provided extra weight, but then you run the chance of braking the implement as it’s not meant for primary tillage). Having said that:
Mouldboard ploughing inverts the soil profile from the ploughing depth upwards, chiseling loosens but barely mixes the soil at all (and isn’t meant to), finally offset discs mixes the soil over the few inches that are penetrating the soil (not a straight up inversion).
I’m not sure what the functional difference is between towed and 3pth versions, aside from the fact that if you have a towed one you typically have to have secondary implement hydraulics on the tractor to lift the wheels off the ground (which I do not have on my old Massey, therefore I use a 3pth version). As for the discs themselves, bigger discs go deeper therefore deeper=more horsepower (your soil characteristics will also play a role in HP required). I like the notched discs personally as they are better at cutting into residue. My offset discs is a 2-gang type, notched at the front gang and smooth discs at the back gang. Love it. I’ll try and attach a picture of it here (see below). I can also change the offset angle so that it can be more or less agressive depending on what I’m trying to achieve and current soil conditions.
I’d say consider the HP you’ll need, (see next point) but also availability! You’re better off buying used discs as new ones can be pretty pricey. So whatever you can find easily could be a good choice, and get some spares because you’ll most likely break a few over time! For your reference, mine are 16" discs.
I’ll attach a handy chart for implement sizing (see below also). For your reference, I have a 50HP Massey 65 tractor and pull a 12 disc wide plough (so 24 discs total since I have a tandem disc plough). I can’t remember the size of the spacers between the discs, but it’s gotta be close to 12in. If you do the math, that means my disc plough is about 10ft wide. I have a fairly heavy soil though (silty loam) and I find that 50HP is enough, but not by much in this type of soil. I expect your 64HP tractor could easily pull my disc plough in sandy soil. Also, check out this in-depth document for some good baseline info:
One implement to rule them all would be awesome, and I would have tried to build myself one if I had the time and skills to do it. I know farmers who have combined the chisel and discs to do all of their primary/secondary tillage. But it almost goes without saying that the more you add on (and the wider it is), the more HP you’ll need. And there will come a point where you’re doing more harm than good (if it can be said that tillage does no harm… but that’s another discussion entirely).
I hope that answers all of your questions! Let me know if you have any follow-up questions.