For an older laptop, that's an okay choice. Ideally it would have more internal storage as 256GB doesn't leave a lot of room for the OS and applications, let alone project files.
To run the current version of Premiere Pro, you want a MacBook Pro that can run macOS Catalina, Bug Sur, or Monterey and the mid-2015 does.
Transcode source footage to Apple ProRes Full Resolution and when needed ProRes Proxy and use corresponding ProRes Sequence Video Preview Settings and you should be good. I'd still export to ProRes and then transcode to H264/H265, but you can export directly to any of the H264 presets from a ProRes Sequence.
For 2160 editing, use the Proxy workflow with ProRes 2160 Full Resolution and ProRes 720 Proxy files.
ProRes 1080 and 720 should work fine.
For storage, pick Thunderbolt 2 SSDs if you can, but you should be fine with USB3 SSD. If you're using Thunderbolt 3/4 storage, the Apple Thunderbolt 3 to 2 adapter needs to go on the drive side.
macOS Big Sur is compatible with these computers
https://support.apple.com/en-us/HT211238
MacBook Pro (Retina, 15-inch, Mid 2015) - Technical Specifications
https://support.apple.com/kb/sp719?locale=en_US