After reading through the nature of your business, I believe quality rather than quantity would help you maintain your reputation; so well paid masterpieces done every now and then should do the trick for you. This would give you quality portfolio to showcase to your prospective clients and also eventually give more value to your projects helping you earn more in the process.

Doing less projects that are well paid, you will still have money coming in and yet be left with ample time to think about business growth and development, which wouldn't be the case of you were doing bulk work for less payments.

About the sales part, I believe that celebrity endorsements generally end up taking the focus away from your brand and showering it all onto the celebrity. Its your ad, so your brand is the star. Focus on the content and on getting the message across to your audience, rather than spending your hard earned money on a celebrity endorsement.

Please feel free to call me if you have any other questions.

All the Best!!