It depends on how big your disc/plow is and how long you want to take to do it.
Many years ago, they used 1-bottom plows and 8n fords or similar. And the average family farm in my area back then was between 100 and 200 acres.
The size of the plow/disc is going to determine how fast you can do the 12 acres. So decide how much time you want to spend, figure what implements are going to acomplish that, and then size the tractor according to the implements.
Current, Power King (antique), Soon to have JD 5103
If you're like me I will get impatient if a job takes too long. I've got about 10+ acres that I cleared this spring and I will be discing this fall. With my tractor I can pull a 6ft. disc and I expect to spend at least a full day to do the 10 acres. If you have more time and patients then a tractor in the high 20's to mid 30's would probably meet your needs otherwise I'd be looking at around 40+HP. Also look at a tractor with a fair amount of weight. If your gound is more or less level then a 2wd will do.
From what you have described, I personally would not want to go below the 40hp mark. 40hp would be the minimum that I would consider. The more hp the better when talking about ground engaging equipment such as plows and discs. You definately do not want to be underpowered when performing these tasks.
OK.
I have a situation similar to yours--10 acres, flat pasture, 6 acres available for hayfield.
Last year I bought a new 2008 Mahindra 5525 (54 hp engine, 45 hp pto, 2WD, power steering, gear tranny, 5370 lb basic weight ) with the ML250 FEL (6-ft bucket). Cost: about $18K plus tax.
The 5525 will pull my old 7-ft Towner offset disc with no problem. Figure about 10-12 hours to disc my 6 acres---multiple passes.
I also have a 13-ft wheel disc (tandem type) that I reduced to about 9 ft wide by removing three outer pans from each of the 4 axles. Haven't tried this one yet, but hope it will reduce the time to 7-8 hours.
My neighbor has a nice 2008 Kubota L4400 (45 hp engine, 37 hp pto, 2WD, constant mesh gear tranny) that would handle the 7-ft Towner. It would probably struggle with the 9ft wheel disc since that L4400 only weighs 3300 lb.
Many an acre has been tilled with the equipment you are talking about, pulled by a Ford 8N (24HP, 3000lbs, and only 2WD). A 40HP 4000lb tractor will do a good job of what you are looking to do. I have a customer who has a Mahindra 2810 4WD (28HP 3000lbs) and pulls a 10' disc with it. I agree that is very marginal but he uses the 4WD, and does get the job done. You should pay attention to the weight of the tractor almost more than the HP. Good luck with your search.
Will this be an on-going operation? (Will you work the ground year after year???) Or is it a one time deal?
Something in the 40hp range and 4000lbs (+) will handle a 2-bottom plow and a 6' to 8' disc fairly well in most cases. Figure 10 to 12 hours plowing, and 3(+) hours discing (per pass) with equipment that size. Does that sound workable? You can go smaller, but time (and possible quality of work) drops off.
Yes, that sounds workable. I'm looking seriously at the Kubota MX 5100 4x4. 50 HP, 44 PTO. The info says 3620 lbs., but with the FEL, I'm guessing I'll be well over 4000. And yes, it'll be ongoing, year after year. So, what I'll want will be a 2 bottom plow - I'm guessing 3' wide and maybe a 7' disc? Is a disc and a harrow/haro the same thing? I'll also probably want/need a blade scrape? Thanks.