When should I consider a Mobile attribution partner? as soon as possible, especially becasue they don't charge for organic installs.
Why? to get a better picture of your traffic sources
What is a Mobile attribution partner ?
mobile attribution is about tracking the source of incoming app installs. To find out which channels of user acquisition work best in the long-term, attribution also covers in-app events that occur after the download (post-install events).
Who are the differences companies?
There are only a few companies in this area as they need to be FB's partners. The leader is Appsflyer. Other companies are Adjust, Kochova, Branch, Tune or Tenjin.
How to choose the right partner ?
As soon as they are FB certified, there are not that much differences between these actors. As most app developers will run FB ads sooner or later, I would not recommend to choose a non MMP partner.
Decisions can be made on very specific topics like service level, pricing, deeplinking capabilities or cross device attribution.
Should I develop my own internal attribution tool?
You could consider it but you won't probably get access to the MMP certification and thus won't get information from Facebook to avoid duplication from installs.
I would suggest to focus your internal effort on building campaign optimisation tools or analysing and detecting possible fraud even if now most of the partners offer fraud detection product.