Set up WHMCS and automatic DNS management

I need a quote, I will create a wordpress multisite installation where people can sign up for blogs and by default, their blogs use subdomains. Wordpress will be on [login to view URL] (example) and when someone creates a blog, the blog will be on [login to view URL] I´m gonna do this and it´s easy for me. But there is a wordpress plugin that allows me to sell domains from inside the wordpress administration panel, when a user creates a blog with a subdomain and when he´s writing posts or something, he can purchase a domain from me and the domain will be set up automatically to point to my server. This plugin works with WHMCS, it connects to whmcs to update the domain dns to point to my server. So what I need from you is this: 1- set up whmcs on a new digitalocean $5 droplet . I only expect to sell like 50 domains in a couple of months and whmcs is only for selling the domains, so we dont need much resources. I already purchased whmcs license. 2- when a domain is sold through WHMCS we can assign a custom name server. As the domain will be used only for connecting to my wordpress, I need a name server with an A record pointing to my server and with the possibility to set MX records from WHMCS in the future because I´m planning to allow my customers to create emails as a premium upgrade. Please provide me suggestions on how we can make this DNS thing work, I dont know if we need cpanel/whm for this... In a few words, I only need to sell domains from wordpress and set the DNS automatically. I´m not selling hosting.