The Marketo Lead ID is assigned by Marketo. You won't know what value it has until the Lead is saved into Marketo. When you use "syncLead" or "getLead", the SOAP response will report the ID.
The ID is not a regular attribute, so I'm not surprised it's not in that list.
Marketo's SOAP docs are... not very rigorous. Expect to do a fair bit of guess-and-testing.
If it helps, here are some sample SOAP request/responses that I've collected, so you can know what to expect:
https://github.com/gbirchmeier/markety/tree/master/example_xml