There's a lot of debate over the ending of Mark, to say the least. There's three main options:

A) The "longer ending" (16:9-20) is authentic and original.

B) That verse 8 is the original ending intended by Mark and the rest is a later addition.

C) There was originally a longer ending past verse 8, but somehow it was lost (or Mark did not intend to end there but was somehow prevented from finishing) and the current verses 9-20 are a later reconstruction.

You'll find scholars arguing for any of the three options. However, option A is very much the minority position and not taken seriously by most, including many conservative scholars. You'll certainly find defenders for option B, but I think option C is the most likely. Verse 8 ends in an unnatural way to be the intended ending of the book, with a conjunction γαρ. It's not strictly grammatically impossible, but it's quite unusual and, to me, not very sensible.

To the broader question: was the resurrection a part of the earliest Christian tradition? To that I give an emphatic "yes". First is the writing of Paul - authorship of some of the epistles is debated or dubious, but some are widely acknowledged to be authentic Pauline 1st century texts; namely, Romans, 1 and 2 Corinthians, Galatians, Philippans, and 1 Thessalonians. These are enough to establish the belief in Jesus' resurrection in the 1st century church, as Paul explicitly writes especially in 1 Corinthians 15 and Romans 6.

Probably the best contemporary scholarly work on this subject is N. T. Wright's The Resurrection of the Son of God. If you're interested in an incredibly in-depth demonstration of the resurrection in Christian tradition, that's it.

The undisputed part of Mark you are referring to does mention a risen Jesus.

^1 When the Sabbath was past, Mary Magdalene, Mary the mother of James, and Salome bought spices, so that they might go and anoint him. ^2 And very early on the first day of the week, when the sun had risen, they went to the tomb. ^3 And they were saying to one another, “Who will roll away the stone for us from the entrance of the tomb?” ^4 And looking up, they saw that the stone had been rolled back—it was very large. ^5 And entering the tomb, they saw a young man sitting on the right side, dressed in a white robe, and they were alarmed. ^6 And he said to them, “Do not be alarmed. You seek Jesus of Nazareth, who was crucified. He has risen; he is not here. See the place where they laid him. ^7 But go, tell his disciples and Peter that he is going before you to Galilee. There you will see him, just as he told you.” ^8 And they went out and fled from the tomb, for trembling and astonishment had seized them, and they said nothing to anyone, for they were afraid.

Mark 16:1-8, taken from the ESV.

As you can see, a risen Jesus is clearly testified, and it also states that he is moving around and talking to the disciples after the resurrection. Having established that the passage in Mark does in fact demonstrate a belief in the resurrection, I do not think there is any evidence to suggest it was not a key motif of the early Christian cult. Mark is the earliest evidence probably written in the late 60s AD, followed by Matthew and Luke in the 80s, both of which very strongly identify with the resurrection tradition, so precluding additional MSS or other evidence appearing, it is unlikely that there is a good argument for the resurrection playing a less significant role in early Christianity.

The question then becomes are the earliest MSS right to leave out the passage 16:9-20, while some other early MSS have it in. Ultimately it is impossible to say what the archetype would be like, and it is possible that the author (or someone close to them) sent multiple copies of the letter with some variation. The contents of this passage is repeated in the other parts of the NT, apart from one line about drinking poison, so it is reasonable to say that the exclusion or inclusion of 9-20 is not that big a deal.

For further reading, anything on the manuscript tradition of the gospels would be worth a read, but try and be aware if it is theological in nature or whether it is purely a textual commentary. A relatively readable and recent edition is Collins' 'Mark a commentary', but your best bet is to see what your local academic institutions and libraries have available.

Among scholars today, pretty much everyone agrees that Mark is written at some point between 65 and 75. There have been some attempts to date it earlier (most notably Casey's Aramaic Sources of Mark's Gospel and Crossley's The Date of Mark: Insights from Law in Earliest Christianity), but most agree upon 65-75.

Mark likely ends with 16:8, as Lightfoot showed over a century ago. It used to be popular to suggest a lost ending (most recently defended in Croy's The Mutilation of Mark's Gospel). Wright's The Resurrection of the Son of God gestures towards a lost ending, but doesn't go all the way in that direction. To answer the question, however:

The first mention of the resurrection appears in 1 Cor. 15:3-8, which is a pre-Pauline creed that likely dates to the mid-late 30s (c.f. Craig's The Resurrection of Jesus or Allison's Resurrecting Jesus, probably the best total treatment of the topic). So, to answer your question, it appears as though there is at least a significant portion of the early Christian movement who believes in a resurrection. Notably, the language seems to imply a bodily resurrection (there's a recent article in NTS about this, I can't remember its title, but I think it's Tour).