Minimum hardware requirements / suggestions

I've used DoggCatcher on my Nexus 7 for about a year; thinking about a smaller dedicated music player. I could buy a used android phone for this. What would you say is the minimum practical android release to run at? I need to have the variable speed feature, BTW. Any suggestions for specific older android phone models that would work for this purpose? I guess SD card support and removable battery would be a plus.