Will your customer or boss give you the time and money to rewrite the code base? More than likely they will not, especially if they believe they have something that already works.
I would suggest coming up with a plan to refactor the code and learn how to black box the things that are less important or too complicated to refactor easily.
It is easy to fall into a trap thinking a new design will be better, but it will take much longer to implement and if the customer/boss is asking for new features along the way it is almost impossible.
I will append to most of the comments here. Describe what you want (describe as many details as you can without writing a book (ex. basic description, technology that will be used and a time frame of when you need it by.) Letting the freelancer know that the a budget has been approved will also help. Many times I've gone through the time and effort of quoting a project only to find out that it was in exploratory means of deciding how much it might cost. Include contact information and best time to call if they have questions.
Then tell the freelancer you are accepting quotes until a certain date and you will make a decision shortly afterwards.
I can do the same if you send me the above information. I can take a look at your project and see if I can help you.