UN is "committed to maintaining international peace and security, developing friendly relations among nations and promoting social progress, better living standards and human rights." The only way for UN to fulfill this mission is to have the possibility to effectively face situations that are considered dangerous or harmful and intervene in case of need. In my opinion UN should then have an army that should be big and prepared enough to stand against the armies of the most powerful UN members. Mechanisms based on balanced debates among UN members should start or stop interventions. This army can be composed of soldiers from member nations.
I think that the United Nations Standing Army is a good idea because it never hurts to be prepared for anything. The Army provides more services than just fighting in war. It provides careers for some people. The Army can also be there to help in the face of a natural disaster.
The UN needs to be ready to enforce its resolutions. You need to be ready for a potential 'mutiny'. The worst case scenario would be a nation openly challenging the UN. They would need to be ready to back up the resolutions they decide on. Maybe setting a minimum on the amount of troops that can be provided for Peacekeeping forces. I'm all for the UN becoming more powerful, although there is a evident American sway that has to be dealt with. But that's a topic for another time. A more powerful UN can be nothing but good in my opinion.
No, a new military stem in the United Nations is not necessary, because the military system in the United Nations is already effective. The United Nations appropriately determines when to use military force, and it contributes soldiers from member nations as appropriate. An overhaul is not needed at this time for the United Nations.