MongoDB Atlas provide 512 Mb of storage space on the free tier. This is good enough to create a MVP. My website https://www.cryptocost.in is using the free tier of MongoDB and I have found it quite beneficial for quick setup.
Further, we were also using it on Production in my previous organization, It provides good analysis tools and performance suggestions for the query executed (though on the Paid tier). You can also upgrade to the latest version of MongoDB easily & hassle-free.
Would recommend using Atlas if you are starting new.
It's always way easier to buy someone else's servers. If you're looking to get something up quickly, Atlas is the way to go. In general, buy don't build unless you _have_ to.