your stock messenger does allow for mms, as soon as you attach a picture(using the icon that looks like a paper clip) to an sms it automatically defaults to mms.
you will never be able to send a message longer than 160 characters as this is the maximum permitted amount that any carrier network is allowed to transmit in one message, depending on which carrier network you are on will determin wether the full 160 character message will be sent as one sms or as multiple sms.
hope this helps
Regards
Baz
