Is there a way to speak "Send" in order to send an SMS?
With Google Now/Voice Actions I don't think there's any way to do this. Some actions will happen automatically after a short delay ("note to self..." is one I know offhand; there may be others), but text messaging requires you to actually press the "Send" button on the card that pops up.

However, you could try a third-party app for these specific functions that you'd like to be completely hands-free. Dragon works a bit more closely to Siri, acting in a conversational manner, and will prompt you for a "Yes/No" to send a message in a very similar fashion. Skyvi is another option you could try, which will send text messages automatically after a 5 second delay (during which time you can press a "Cancel" button to stop the sending).