MaxCDN is a different kind of CDN, which can be used in conjunction. They cache objects at the edge before the request arrives, Cloudflare does not.
You can use Cloudflare to reduce the usage (especially if configured correctly) on the MaxCDN endpoint.
Also Cloudflare has a bigger footprint than MaxCDN.
As for the SSL certificate you would still need one for the origin so that ideally you would have an encrypted connection to Cloudflare and then from Cloudflare to the origin. This is the Full (Strict) mode you see in the Crypto tab of the dashboard.
The certificate can be either bought (not recommended), done using Let’s Encrypt or using one of the Cloudflare Origin Cettifcates you can get from the dashboard (which are valid only from Cloudflare’s edge nodes and won’t work for actual users).