The app platform has a load balancer built into it, allowing you to scale the number of instances of your application to your needs. That being said, could be times when you would like something like pm2. Some of the instance sizes offered have multiple CPU cores, and to be able to take advantage of them you would need to either have multi-threaded code, or spawn multiple worker processes. There are multiple ways to achieve that. Also, if the app platform will try to restart the application if the process dies.

It is possible to add connections to redis or other databases. DigitalOcean offers managed databases (redis, mysql, postgres, mongodb) which can be used with the app platform.

Pricing depends on the scale of the application, plus any overage fees for extra bandwidth or excessive building times. Managed databases are also an additional service that would add extra charges. Pricing for app platform can be found here. Pricing for managed database can be found here.