When you request an HTTP website (for instance, http://example.com), you make a connection to port 80 on example.com's server. This does not use port 80 on your local machine.
It follows that if you want to run an HTTP server on the traditional port at your house, you need to use port forwarding to tell your router to forward any requests on port 80 to your server. This will not in any way affect your traffic out to other servers' port 80s or expose you to a MitM.
There are many people around the world who are constantly hitting every possible IP address and trying common ports for vulnerabilities. They have been trying to get into your Digital Ocean server, and they will try to get into your home server. The difference is what happens if they succeed: on one hand, your DO machine gets temporarily turned into a spam machine (likely) until DO notices and shuts it off, while a breach in your home system can not only do that (which is why most ISPs block outgoing mail server requests) but potentially gain access to your internal network and all your other computers.
It is not necessarily a bad idea to run a small webserver at home. But you should learn about common hardening techniques (more than I can go into for an answer here), keep it up to date with security patches, and actively monitor it for breaches.