Will people that only believe that there is one God and no one is equal to him eventually go to heaven? After experiencing hell, of course. As there are many people that I know that don't follow a specific religion (but are not atheists), but believe in the oneness of God. Basically tawhid! And compare him with no other. Also there are some people who were born in Christian families, but they do not compare Jeus to God. What about them!?
Belief in Allah is not sufficient to bring you into the fold of Islam. It must also be accompanied by the worship of Allah alone. The disbelievers from the Quraish believed in Allah, but they were still coined disbelievers because they used to worship idols along with Allah:
قُلْ مَن يَرْزُقُكُم مِّنَ السَّمَاء وَالأَرْضِ أَمَّن يَمْلِكُ السَّمْعَ والأَبْصَارَ وَمَن يُخْرِجُ الْحَيَّ مِنَ الْمَيِّتِ وَيُخْرِجُ الْمَيَّتَ مِنَ الْحَيِّ وَمَن يُدَبِّرُ الأَمْرَ فَسَيَقُولُونَ اللّهُ فَقُلْ أَفَلاَ تَتَّقُونَ Say (O Muhammad), "Who provides for you from the sky and from the earth? Or who owns hearing and sight? And who brings out the living from the dead and brings out the dead from the living? And who disposes the affairs?" They will say: "Allah." Say: "Will you not then be afraid of Allah's Punishment (for setting up rivals in worship with Allah)?" Surah Yunus (10) ayah 31
So we must also worship him alone along with believing in Him. The disbelievers affirmed Tawhid Al Rububiyyah (Oneness of Allah's lordship) but they denied Tawhid al Uloohiyyah (Oneness of Allah's worship)
As for non-Muslims who believe in Allah alone:
وَمَن يَبْتَغِ غَيْرَ الإِسْلاَمِ دِينًا فَلَن يُقْبَلَ مِنْهُ وَهُوَ فِي الآخِرَةِ مِنَ الْخَاسِرِينَ And whoever seeks a religion other than Islam, it will never be accepted of him, and in the Hereafter he will be one of the losers. Surah Al Imran (3) ayah 85
This means that they would also have to affirm Tawhid al Uloohiyyah according to how Islam teaches us to worship Allah otherwise they may be of the losers in the hereafter.
Heaven and Hell are to be decided by Allah and we should let Him do that.
Regarding the fact that what's enough and what's not? In my humble opinion, doing your best is what's required. If you think you're capable of doing more and intentionally avoid from doing so, then you have to answer that too to Allah.