In theory, every monitor can be hardware calibrated using a colorimeter and its associated software. The 2012 released U2412M was not validated with our Dell calibration software. You would have to use 3rd party calibration hardware and software. The DUCCS (Dell Ultrasharp Color Calibration Solution) was validated with the X-Rite i1 Display Pro colorimeter for the following monitor models =
UP3218K, UP3216Q, UP3214Q, UP3017, U3014, UP2718Q, UP2716D, UP2715K, U2713H, UP2516D, UP2414Q, U2413

1- All monitors are calibratable when plugged into a computer with a graphics card that has LUTs (all of them right now)
The name "hardware calibration" is used for monitors which have user-programable LUTs inside them. For example the ones listed by Chris if yiu want a Dell
The name "software calibration" is incorrect, since graphics card calibration for those monitors without "hardware calibration" relies on the existence and precission of a dedicated hardware in graphics card = LUT. All cards are not the same, AMD graphic cards used dithering at LUT output so you could get a smooth calibration on gradients even without a monitor with hardware calibration.

U2412M is old, sufferrs from PWM flickering and low brightness and have a very dirty grained mate coating like U2410. If you want a U2412M get the new version U2415. There is no reason to buy the old model... I didnt know that you can purchase the old model right now.

2- Benq is a bad choice for widegamut, the same uniformity issues like widegamut dells and worse software. They are in Dells price range. I think that you meant to say Eizo or NEC which are more expensive, but just a little more (more expensive= better color uniformity across the screen for the same screen size).

3- Answered in 1), Graphics card with dithered outout (AMD) could do the job.

Depending on CYMK profiles you use, a sRGB monitor like U2412M/U2415 won't be able to show certain kind of cian turquoise printable in offset printing.

So your question may be wrong. You may want a monitor whcih can show all the colors you may want to print : widegamut (AdobeRGB), and maybe this is the question you should ask.

Widegamut monitors with good color uniformity are expensive. Cheapest 24" is about 650 euro (Eizo CS2420). There are cheaper monitors but it's like flipping a coin about their color uniformity across screen (green or pink tints). Without a big budget, I won't go to a widegamut, too risky. Stay with a sRGB monitor like U2415 and if you wish hardware calibration so much, NEC EA245wmi it's just 100 euro/dolar more expensive.

Aso you'll need a colorimeter for calibration, i1DisplayPro about $250 or something like that. Don't go to Spyders, not worthly. If you have no money for a good one save money and go to i1DisplayPro.

yumichan, as always, thank you for your expert advice.