Aero EPC-9 help

I’ve had an Aero EPC-9 stripped lower for a while and finally able to start building it out. What I don’t know is what issues to be aware of and recommendations around said issues. Worthwhile upgrades are also welcome. The biggest help would be if anyone knows of a good alternate to the upper receiver. I haven’t been able to find an upper in stock anywhere and it’s rare that I have the time to do a deep dive looking into compatible parts. If anyone could give some guidance or even a build list, I’d be greatly appreciative.