Excel could certainly calculate your payments and print a bill of sale, but it is a horrible task manager. I would look at an Office 365 business plan, which is around $12.50/mo per user, and includes Excel, as well as Planner, which is a task management solution to either alone or shared with team members.
You can get Invoicing for Small Business too.
I love Excel and use it daily, constantly, but I know it isn't the end all solution. Use the right tool for the right job.
But yeah, you should be able to get out of the $3K/mo software you are paying for. That sounds absurd for a business your size.