You definitely won't be able to get a professional grading monitor for 400 Euro. Those cost at least 4,000 Euro, if not more. You probably won't even be able to get a decent 4K TV for 400 Euro.
But if you're serious about getting into color grading, I would consider looking at used Panasonic plasma HDTVs from the last few years. Those have the ability to produce rich blacks and the color can be quite good after calibration. I have a Panasonic Viera ST50 from 2013 that was around $1000/new, I think. Just looking on eBay, more recent used models are around $400, so under your budget.
It works well enough for home grading on my Mac, along with a Blackmagic Mini Monitor and Davinci Resolve. The Mini Monitor (which converts the video on your computer to broadcast standard video for proper viewing when used with editing and color grading software) is $150, and Resolve can be downloaded for free. That's about as cheap as you can get, I think.
But for grading, I think you actually want a TV or broadcast monitor with HDMI or SDI. The reason is that computer monitors operate in a different color space (sRGB, Adobe RGB, etc) from broadcast video monitors (Rec709). So in order to accurately preview how your work will look on TV monitors and home video projectors, you need to work in Rec709 color space.
Editing programs like Adobe Premiere and grading programs like Davinci Resolve have those proper video outputs available to you, as long as you use a device like a Mini Monitor or an Ultrastudio HD to convert the signal. You connect the device to your computer's Thunderbolt port, and the device to the TV monitor with an HDMI or SDI cable. Then the program recognizes that there is a video monitor attached and outputs a proper Rec709 video signal to that monitor.
If you just plug your TV or computer monitor directly into the computer via HDMI, then the computer will simply treat it as a second computer screen and not convert the signal to broadcast video standard. Which is fine for editing when you simply want to have a larger workspace. But if you want accurate looking video, you need to pass through a conversion device first.
So just using any computer monitor with an HDMI port is a bad idea for color grading. You can use that monitor for your GUI or as a second screen for more work space, but you can't rely on the color or contrast to be accurate.
Thanks a lot now i understand why i need a Mini Monitor or an Ultrastudio HD it's very clear, actualy i'm very serious about getting into color grading "Davinci", and i'm gonna spend more money to buy a good monitor with a mini monitor or an Ultrastudio HD, the LG Ecran LG 31MU97Z - 31" is a 4k 4096x2060,10 bit monitor with ips Adobe RGB over 99.5% / DCI-P3 97% and thunderbolt, the price is 1000 euro.
1-Do you think is a good idea the buy this monitor ?
2-do i need the mini monitor or an ultrastudio even if the monitor have already a thunderbolt port ?
I don't know anything about that model of monitor, so unfortunately I have no idea. But yes, you'll still need the Ultrastudio because it's not about having compatible computer ports. It's about converting color spaces and getting a broadcast legal video output out of the computer.
One of the 4K models, I imagine. You would also have to check if the free version of Resolve actually outputs 4K video. Again, the LiftGammaGain forum would be a good resource for these types of questions.
Honestly though, you don't necessarily need to monitor your edit and grade in 4K just because your camera shoots 4K. Monitoring in HD is still a lot more common, as higher resolutions tend to eat up a lot of graphics card resources for minimal benefit, especially once you are working with a lot of nodes or layers. Personally, I don't think monitoring 4K makes a lot of sense unless you're regularly delivering for theaters and other large displays.
You don't need 3 grading monitors. One grading monitor and one for interface is enough. Later you might get another one to display scopes, but now getting an adequate grading monitor is a first priority.
Seems adequate. But you should better ask at LiftGammaGain as I know nothing about LCDs and neither do most of us here. Basically you need a monitor which will precisely enough display colors within Rec709 spec and which you can calibrate to a '709 gamma.