I have a wealth of experience developing for WordPress and can have your project completed for you within 3 days of accepting bid.
If the functionality is not already there in the autocomplete plugin, it won't be an issue for me to change the form input it is bound to. Adding a minimum number of characters validation method and displaying how many characters have currently been typed I would do using JavaScript on the front-end, then on form submission use PHP to double-check that there are indeed the correct number of characters, returning the user to the page and showing an error message if not.
If you have any questions, please don't hesitate to contact me.
Regards, Jake.