I'd recommend doing things by yourself as well if it is for money. Before arriving to Korea, looking for maps and how subway works, places to stay, convenient stores, supermarkets etc is easy now with google
The only thing is language barrier, but if you learn a few sentences and expressions you should be fine. And in Gangnam there is always people that can help you. We're in Gangnam after all, if you walk in shops and ask for help, or people in the streets they would be happy to help
Doing things with docfinder or any other agency is very convenient as you don't have to do a throrough research. They take you everywhere, some agencies would help you shopping for the food or medicine you would need and even do a free Seoul tour. And after surgery, with general anesthesia, pain, uncomfort and all + the language barrier... you mind find helpful people dealing with everything for you
Personnally I'd do everything myself. Because... I don't even know lol maybe more because of money... So I didn't even bother contacting DFK. I kind of feel pressured with someone always with me. But I agree that with surgery it would be helpful when you don't feel your best and those who are super scared of language barrier