I setup Salesforce for larger companies as a major part of my profession. I also consult with a lot of startups and small businesses and tend to steer them away from Salesforce for the time being. A SUPER BASIC Salesforce setup with a consultant will cost you at least $5,000. What you want to do is more in the tens of thousands (or more) unless you are lucky and a tool actually does what you want.
A lot of companies try to solve their business problems with a tool, but sometimes you need to start with just streamlining how you run your business and standardizing how you run your business. The more complex or unique you build your business the more money it will cost later to customize Salesforce to your needs.
What you might actually want to look into is a WordPress Membership management tool like iThemes Exchange or WooCommerce. You can have clients signup up for different recurring membership levels and have an account and then assign them a key. Possibly you could store this key in their product info so they could login to retrieve it. These solutions have integrated payment processing and will integrate with Quickbooks Online, etc. you might have to do some work though to ensure you can confidently store activation keys in WordPress securely.
If you are interested in going down either path and want to talk, let me know. I have a site called GrowthHeroes.com that helps people DIY websites, CRM, marketing automation, etc. It's still in the early stages but has some free public content.