I'm, I'm pretty sure that's not actually the plot of The Hand of Fear. Eldrad regenerates through the use of radioactivity- first from the microscope, then the reactor and finally on Kastria. It's cellular regeneration, not necessarily with bodily change- he patterns himself on Sarah at first, but later adopts his "true form" (male) before dying. I'm struggling to remember any absorbing of the Doctor's DNA or becoming "half timelord".
SBT
SBT