As far as patient safety i have each electrode connected to 220 ohm resistors.

The impedance of the human upper body is around 2 kOhm. Another 220 Ohms is pretty much meaningless when you have an electrical fault that places line current on one of the electrodes.

What about galvanic isolation of the circuit (though there are some pitfalls here, especially with transformers, that may make the circuit galvanically isolated but still unsafe), or running it off batteries ?

is an 8 bit adc suffice???

Um ... that mostly depends on what offsets, dynamic range and resolution you want to display. Maybe you should start with writing down some specifications instead of worrying whether some piece of hardware will meet your nonexisting/unknown specifications ?

Depending on what kind of filtering you do in the analog path and whether you're content with a very coarse resolution, an 8-bit ADC might, just might be sufficient to produce a recognizable ECG wave - but don't expect it to be useful for any kind of advanced diagnosis.