Hello,

You can add a drop down list and the CMSRepeater controls to the web part:

<asp:DropDownList runat="server" ID="ddl" AutoPostBack="true" OnSelectedIndexChanged="OnSelectedIndexChanged" ></asp:DropDownList>
<cms:CMSRepeater runat="server" ID="rep" TransformationName="cms.news.default" HideControlForZeroRows="true" ></cms:CMSRepeater>

Within the Page_Load method, you can bind the drop-down list control with data set as shown here:

TreeProvider tree = new TreeProvider(CMSContext.CurrentUser);
DataSet ds = tree.SelectNodes(CMSContext.CurrentSiteName, "/%", "en-us", false, "cms.news");

if (!DataHelper.DataSourceIsEmpty(ds))
{
    ddl.DataSource = ds;
    ddl.DataTextField = "DocumentName";
    ddl.DataValueField = "DocumentID";
    ddl.DataBind();
}

The drop down list control provides the OnSelectedIndexChanged handler in which you can access the selected DocumentID, get the TreeNode object and bind the CMSRepeater control:

protected void OnSelectedIndexChanged (object sender, EventArgs e)
{
    TreeProvider tree = new TreeProvider(CMSContext.CurrentUser);
    TreeNode node = DocumentHelper.GetDocument(ValidationHelper.GetInteger(ddl.SelectedValue, 0), tree);
    rep.DataSource = node.GetDataSet();
    rep.DataBind();
}

Anyway, you should also consider caching returned data set of documents to improve performance: Caching API examples.

Moreover, more information about developing web parts, can be found here: Developing web parts

I hope this will help you.

Best regards,
Michal Legen