Invitations have come a long way. There are so many different styles and budgets and this really comes down to personal preference.
Save the dates are great to send out as soon as you have your venue and date chosen, especially if you have a lot of family and friends traveling from out of town. Are they necessary? No, but typically much appreciated and these can be simple postcards.
Invitations should be sent with a way to RSVP to the wedding. You will need a head count for catering, table set up, etc. You can either send an RSVP card with your invitation that will need to be mailed back to you, or you can set up a simple website in which your guests can RSVP directly.
Things to consider: