I am an agnostic who has come to learn about Islam, and I now believe in one God. I also think that Muhammad could have been a prophet, but I still have a few doubts about this. I would say I am about 60% sure that he is. Should I hold off on taking my shahada? I also don't know how strictly I would adhere to the rulings until I am completely convinced but I feel as if I will never know for certain, as I have a nature to doubt.
Welcome to Islam and this site per se. First of, I would start thanking God for the food I eat and the things I possess. I think for your Shahada (bearing of witness) would look good and seem 100 % satisfying to you, when you believe 100 % in both the halves of Shahada. There is no rush. The role of any prophet was to to introduce us to God and prescribe a way of life.
Take your time to understand and believe. As others have suggested, take time to read and understand the Qur'an, and other getting started articles like may be this. There is nothing wrong in being curious and, feel free to come back and ask any questions you have which you can't find answers to.
Testimony is said to testify to the oneness of God and the mission of the Prophet Muhammad (peace and blessings of Allaah be upon him). The Qur'an and the Sunnah of the Prophet of Islam (PBUH) and the Imams are the main source for understanding the religious beliefs and practices of Islam. According to Muslims, there is no falsehood in the Qur'an and it has remained undistorted since its revelation. Tradition also includes the words and deeds of the Prophet and the Imams, which have been passed down in writing from generation to generation.