OWL, how do I...

While writing a simple OWL ontology for describing some RDF data, I
stumbled over the following three issues:

1) How do I state that any 'Measurement' must be an integer and may have
one 'unit'?

  <foo> <rdf:type> <Foo>
  <foo> <length> <#_1>
  <#_1> <rdf:type> <Measurement>
  <#_1> <rdf:value> 42
  <#_1> <unit> <Meter>


2) How do I state that any statement with a 'length' property is
expected to have an author property?

  <rdf:Description rdf:about="urn:test:1">
    <length rdf:ID="#_1">42</length>
  </rdf:Description>

  <rdf:Description rdf:about="#_1">
    <author>Me</author>
  </rdf:Description>


3) What's the benefit of using owl:Thing instead of plain simple
rdf:Description for describing data?