If my car requires an after market dash panel, cause my heat/ac is connected and tied into my top screen.
It's 800 for good head unit and dash/steering wheel kit.
Would leaving the factory head unit (i do believe it has a small amp under the drivers dash for amp out, so no converter needed i believe)
be okay for quality sound if I decide to put a system in my car?

well the factory stereo will always be the weak point if you keep it. if you are wanting to keep the factory amp as well then really there's no pointing in replacing any of the speakers unless they are blown.

realistically you need to replace the factory amp at least to make any of this worth doing.