Welcome!
Hello and welcome to the forum. I am glad you have been inspired to learn more about Islam. I think you are asking some important and good questions. Turn to God with a sincere heart and ask for guidance and most surely he will guide you. God is most Great and does not fail those who put their trust in him!
In response to your questions, I have some other questions to ask you that might help you start to sort these things out.
What do you believe about God? If you had to, how would you define God? And how have you come to this belief? Have you ever read the Qu'ran? If not I would suggest you try reading it. It is stated in the Qu'ran that those who are guided recognize that it is the Truth from the Lord.
[22.54] And that those who have been given the knowledge may know that it is the truth from your Lord, so they may believe in it and their hearts may be lowly before it; and most surely Allah is the Guide of those who believe into a right path.
[28.53] And when it is recited to them they say: We believe in it surely it is the truth from our Lord; surely we were submitters before this.
Also, it is very important to be able to distinguish a real from a false prophet of God. Do you currently believe there have been any prophets that have lived here on earth? And if so, who were they? And why do you believe that?
Personally, I do recognize that there have been many prophets of God, and that they have all come with the same message. The basics of which is to Worship the Lord Almighty, alone and without partner, and to do good and shun evil. If someone comes with a message that deviates from this, I would not consider that person a prophet. In the Qu'ran we are told that God has sent prophets to all nations as a warning and call to return to the worship of God. Not all of the prophets are mentioned in the Qu'ran. But those mentioned in the Bible such as Adam, Abraham, Noah, Moses, David, Job, Jesus, among others, are all mentioned in the Qu'ran, and we can have confidence that their stories contained in the Qu'ran are accurate.
The revelation of the Qu'ran to Muhammad, may peace be upon him, was much like the revelation of the Torah to Moses, may peace be upon him. And the example of the prophet Muhammad's life is so similar to the example of the prophet Jesus's life, to me it almost looks like an extension of the same life they presented such a similar example. It is an unfortunate thing that the Bible has been altered. Because there is truth and important stories contained therein too, but with the alterations that have been made to it over the years, it becomes risky to rely on those messages completely. It is such a shame and a huge loss. Fortunately for us, in God's great wisdom, he sent the Qu'ran to rectify this.
In the Bible there are prophecies of future prophets to come. Jesus was the fulfillment of the prophecy of the Messiah. And there was another prophecy of a prophet to come from the descendent of Abraham's son Ishmael (recorded in the book of Deuteronomy). Even Jesus made mention of the one who would come after him known as "The Comforter", who would verify everything Jesus had taught and verify the fact that he was the Messiah. Muslims recognize that it is Muhammad who is the fulfilment of this prophecy, and that he is indeed the seal of the prophets, and what many refer to as "The Universal Mercy". I encourage you to learn as much as you can about the example of this great man and draw your own conclusion as to whether you think he is a prophet or not.
[7.157] Those who follow the Apostle-Prophet (Muhammad), the Ummi, whom they find written down with them in the Taurat (Torah) and the Injeel (Gospel) (who) enjoins them good and forbids them evil, and makes lawful to them the good things and makes unlawful to them impure things, and removes from them their burden and the shackles which were upon them; so (as for) those who believe in him and honor him and help him, and follow the light which has been sent down with him, these it is that are the successful.
There are others on this forum who are more well versed in addressing some of your other questions regarding the history of the recording of biblical passages and how they came to be altered, insha'Allah you will receive some responses that will better address those questions as well.
I commend you in your search for truth and hope you will find much success along your path in life. Best wishes to you!
on your post. Absolutely, the clear answer is in the Quran, to be sure.