Well I wouldn't consider buying one but not for any bad reasons.
My reasoning is that I do both plumbing and drain cleaning out of a 3/4 ton van and there is a limit to what I can carry.
I have 2 drum machines that pretty well cover the spectrum of my drain cleaning work and I'll stick with those with my space and weight limitations.
As far as buying the Electric Eel Model C used there is not much that I would worry about. It is a very robust appearing machine and very simple as well. I'd say it bears a close resemblance to a brick privy. As long as the motor runs, the gearbox is good without excessive gear grinding and bearing noise, I'd say there is little to worry about.
I'd be more concerned with the condition of the cables if they were included as part of the package.