I've bought a pair of Beyerdynamic DT 770 PRO 250 OHM headphones for the purpose of producing, mixing and mastering. I don't know if I'll just need an audio interface, or amp/dac or something else. What product/combination shall I buy? Thanks
250 ohms is a high impedance for headphones. If the headphone output of whatever you are connecting it to is inadequate they won't play loud enough but it won't cause any damage to try it.
If you need to drive them then what you use will depend on the PC rig. If you don't need the other functions of an interface then you need at least a headphone amp. For better quality use a DAC + headphone amp or combined DAC-Amp.