'65 Cuda Fastback
I agree- $5,000+ would need some real proof of some good engine work.
As for non-original (racing) seats and a CD player and non-original gauges, this in most people's opinion would bring DOWN the value of the car. You mentioned speakers, are they in the doors with holes cut into the interior? If so, this would also bring down the value to most. On the other hand, I do realize you might not care about originality as most as many of us - so these things might ADD value in your opinion - which is totally acceptable- just want you to realize what your getting yourself into. Thinking of the resale value of a car is also important when buying something.
Whatever you do - find out as much as you can and let us know the details. My first car when I was 16 was also a 65 B'cuda - and the thing I regret most was not finding out more information and making SMART decisions.
As your questions about power steering and A/C - it is much more likely to have power steering than A/C - however IMO the power steering in the old mopars was pretty crappy. Don't worry about manual steering, it will just build you some better muscles.