Is it possible to take good quality portraits using just the camera in a well lit room or would I need to buy lighting?
It's every bit possible. More and more lately I've taken to disliking obviously artificial lighting (totally personal preference). I don't mean the use of it, I mean the lighting in a shot looking totally unnatural. Underexposing the background too much by using a powerful flash for key light kind of vibe, I just don't like it.
Having started with flash, and worked with and owned various Elinchrom and Bron gear, having assisted fashion and commercial photographers shooting every other system out there pretty much, I have grips with it and I'm comfortable with it. However, lately I've had the sudden realisation that my natural lighting techniques reeeeeeally lag. Or were. So I'm currently shooting solely natural light. Indoors, outdoors, wherever, and at any time of the day.
I'm enjoying shooting now more than ever and in a whole "new light"

lol.
You need nothing more than your camera and a fast lens (low f number). If you want to add light, get a handheld reflector, if you want to take it away use boards painted black, or even better covered in black velvet. For effects use things like mirrors, reflective surfaces, glass, vases with smashed mirror around them, hell, cover half your lens with an ash tray. There's an infinite amount of possibilities with natural light, just remember not to be afraid of higher ISO, and practice your grip for slower shutter speeds. I handhold an RZ67 with 180mm lens at 1/15s, practice practice practice!