I don't think you will find a legal requirement for an apartment block to have a security front door. You will find that many apartment blocks don't have a security door.
It would however be a desirable feature for buyers. Most real estate ads mention “security door” in their list of features. I personally would prefer to have a security front door. They do provide security – except when residents prop them open that is.
If you should want to push this matter, do consider that you would have to have an intercom system installed as well so that your visitors can get in. And thats where the cost would be. I don't know how much it would cost to have such a system installed but do know that to have a system replaced costs quite a bit.