An open back will help a lot. Large holes in the bottom to let air in would be good, particularly at the front of the cabinet, to encourage airflow from the front bottom to pass over the gear and out the top back. But what, precisely, is needed depends on the dimensions of everything, and the particular gear you are using. Some of it generates more heat than others for the same output. You probably want to go with at least twice as much space around the gear as the manufacturer recommends, given the front, sides, and top will be closed off. In other words, I recommend placing the gear inside in a manner that seems to waste a lot of space, as that "wasted" space is there to allow for the heat from the gear.
If you own the place, you could also consider an in-wall installation of the components, depending on what the room is behind the wall. Basically, you cut a hole in the wall, put the gear on shelves in the other room, so that the front of the gear is at the wall. You then leave plenty of air space around and above the gear in the back room. You could have nice wooden doors in front of the gear so that you would have a very minimalist look. Obviously, this depends on the back room not being used in a manner that is incompatible with doing this.
I am thinking about a slot hole in the front, under the vertical front wood of your picture (which I notice does not go down to the floor), that draws air in from the room up front rather than from behind. The bigger this can be, the better. In fact, if you can just have the bottom shelf not go out to the front door of the cabinet, that might be best. Maybe a 3 cm gap if you are okay with it. This, I think, would help with the airflow. You might also want other venting in the bottom, depending on how the rest of the cabinet is being made. I should also tell you that I am not an expert on the subject of airflow, and that you might want to get a second opinion from someone who is. If you have room, you might want the receiver on the bottom, with no shelf above it, so it has a lot of space above it to the top portion of the wood. The reason being, 550 mm is not a lot of space for shelves when you are also concerned about heat and have the front, top, and sides closed off. The BD player can be placed beside it, though it can be in another section if you need a vertical support for the top surface of the cabinet. This will take more horizontal space than you may be wanting to use (because I am suggesting that you use the entire vertical space just for your receiver), but it will help with your heat issue and probably make it okay, assuming you do keep the back open and do everything else I have suggested, and don't try to do something crazy, like put tube equipment in there with it.
I personally hate fans, as they can make noise, and are something that use electricity and can malfunction. So I recommend being generous with passive solutions instead.
As an aside, I have never had a problem with gear overheating. But I am someone who understands that when a manufacturer puts vent holes in equipment, that they mean for them to not be blocked. And I pay attention to warnings about how much space there should be around the gear, as per the manufacturer's instructions in owner's manuals. I suggest you pay attention to that sort of thing as well, and read the owner's manuals now, even if you have not yet purchased the equipment that you are planning on using.
As another aside, I like real wood, and I like solid wood, rather than veneer, for cabinets.
I would definitely not put the amp at the top of the cabinet. Heat rises, and so without a lot of airflow, it will get warm at the top of the cabinet. I recommend that you put the amplifier on the bottom shelf, and not have a shelf above it at all, so there is plenty of room above it for the heat to rise. (In other words, with your removable shelves, don't put in the shelf above it; have nothing between the top of the amplifier and the top of the cabinet.) The BD player can go next to it, or in another section of the cabinet. And, again, I would want a gap in front, so that the bottom of the cabinet does not meet the front door, but instead has space for plenty of air to enter which should naturally tend to come in, as air at the floor is cooler than air at the top of a room, unless something is heating the floor. Which means, of course, that if you have underfloor heating, you will want to do this differently; my advice is predicated on the idea that you have some other form of heating, like forced-air, steam radiators, etc.
Perhaps I am not being clear with my idea. It would not be visible with the door closed. It would only affect the look of the cabinet while open. And then the front of the bottom shelf would simply be further back than the top and sides; otherwise, it would look like a normal shelf. The idea is to have air from the front to go under the door and up into the cabinet space. Then it will go up and out the open back. Thus, you have cross ventilation for your gear, to keep it cool.
Now, from looking at your picture again, it is unclear whether or not there is a vertical piece on the front below the door that is recessed (the "kick" area), or if there is no such piece. If there is, my idea is to have the front of the bottom shelf end at the point of the recessed vertical piece, so its top is still covered by the shelf, but the shelf goes no further forward. If there isn't such a vertical piece, such that there is nothing (other than your casters) under the bottom shelf, then you could instead just go with large slot holes on the bottom, starting close to the front. Either way, I would not put a shelf over the amplifier, and give it plenty of space above it. If, over time, you find that you don't need that extra space for heat dissipation, you can then put in your removable shelf and use more of the space. But I would not count on being able to do that.
If you read the rest of my post, you will see that I recommend having nothing above it. Consequently, there could not be anything else above it that is being heated.