What can ECG DC level be?
This discussion is on DIFFERENTIAL ECG signals.  There can be common mode signals of volts.

Output DC levels are zero, as even the widest ECG high pass filters at 0.05 Hz.  On the input, that's going to depend on electrode offsets, skin preparation, and other electrode attachment factors.  I always allow for at least 150 mV. 

That's the target for a working ECG.  The clinical units are built to survive relatively low frequency transients of hundreds to thousands of volts in the event of defibrillation.

When you see large low freq offsets in ECG output, you're usually talking about bad things, like fibrillation, and I'd put possible offsets at around the magnitude of the QRS complex.

As for common mode signals, Winter and Webster, 1983 (link to pdf) say it can be as small as a few millivolts if the subject is well grounded, and reach 20V if the subject is holding a power cord.  It can, of course, be reduced if using driven-leg techniques (link to pdf), but that won't work at 20V because of amplifier saturation.