The Raspberry PI 4B with 8 GB of RAM. The performance gain is small for gaming because RAM is usually not the bottleneck
Eh? The performance gain past 2GB is zero. Retropie uses around 1GB with swap and all. The extra ram might help with installing emulators from source or if you want to use it as a desktop someday.
You need a compatible power adapter and lots of companies make them.
I only recommend the official psu due to reliability. They are $8US so there shouldn't be a reason to get a 3rd party psu that no one knows how reliable they are.
If you're going with a plain pi4, for a case, I recommend the argon one v2 if you don't want that nespi. It has a fan and converts the microhdmi ports into normal sized ones.