OK, a few things here:
1. There is probably no ADC chip which will talk to the RS232 serial port itself and do what you want to do.
2. The serial port is probably not even capable of reading 50kS/sec. At 16 bits/sample, that would need 800 kBaud whereas 57.6kBaud is the best I hear about. So if it's like an oscilloscope then the hardware needs to handle the signal. Trigger, read a screenful of like 256 samples and save it to memory, then just transmit that frame of samples. A microcontroller can do this.
3. Isolation. The controller and its ADC, as well as any external ADC you might add, are going to share a ground with the serial port and the computer. This may or may not be acceptable. If not, you'll need to isolate the ADC section with an optocoupler and come up with an isolated power source for it (battery or an isolated DC/DC). Unfortunately, optocouplers and generally fairly slow. You probably could not capture at a rate of 800KBaud through any opto. You'd need to add a microcontroller on the isolated side with the ADC to talk fast to the ADC, capture a burst of ADC readings, and then feed them through the optoisolater to a microcontroller on talking to the serial port. Or probably you could just make the isolated ADC microcontroller talk through the opto straight to the interface chip (like a MAX232).