How to create a dropdown list with node ids?
To get all the nodes ID's for a content-type, you can use EntityFieldQuery and node_load_multiple:

$query = new EntityFieldQuery();
$result = $query-&gt;entityCondition('entity_type', 'node')
  -&gt;entityCondition('bundle', 'mycontenttype')
  -&gt;execute();
$nodes = node_load_multiple(array_keys($result['node']));


If you are using FAPI (form API), you can populate your dropdown list with these values easily.

Unfortunately, I can't help with the last part (displaying the node under the form).