It does not matter which firmware was originally on it, any firmware will do. As for a specific version, its better to use the newest version because its supposed to have bug fixes and improvements - thats why it is a newer version.
Each firmware has a configuration file(s), some comments in that file will be explanatory. But there is a some stuff to tweak and adjust, like thermistor tables and extruder steps/mm. These ideally have to be tweaked and adjusted individually on a particular machine itself. If you can ask the seller about these it would better to have these than just firmware version. But i dont think it would of worked with that, perhaps he/she just used some generic data instead of tweaking it, so perhaps you wont get anything useful. I dunno. There will be other info necessary like axes steps/mm and such, endstop configurations, uC pinouts, but these ones and rest of things like that dont need any adjustment/calibration, so these are fairly easily to figure out.
Each firmware has a configuration file(s), some comments in that file will be explanatory. But there is a some stuff to tweak and adjust, like thermistor tables and extruder steps/mm. These ideally have to be tweaked and adjusted individually on a particular machine itself. If you can ask the seller about these it would better to have these than just firmware version. But i dont think it would of worked with that, perhaps he/she just used some generic data instead of tweaking it, so perhaps you wont get anything useful. I dunno. There will be other info necessary like axes steps/mm and such, endstop configurations, uC pinouts, but these ones and rest of things like that dont need any adjustment/calibration, so these are fairly easily to figure out.