I can see where you're coming from, since I am a student myself (working on my masters thesis in film/animation) but I don't agree with your assumtion that "limit your workload" as I was suggesting means that you're taking away the challenge. Funny, I was thinking about this comment I left last night as I was reading a book for my work. I ran across some info in it that described what I was trying to say:
"There is nothing wrong with challenging yourself, just be sure that when you begin the process you have factored in your abilities and the amount of time you have to complete your task." - 3D Character Setup by Michael ford and Alan Lehman pg 56.
A challenge is good and something you want, but you have to be realistic about your time frame and constraints. What is the goal? Is it to create a rough animation? To create a finished animation? To learn facial animation? Action animation? Learn to convey emotion through gestures? Since the goal here seems to be to create a full animation, start to finish, with one bipedal character I assume the main focus is on conveying the character through action. To do this with the ferret would take more time than it would with the doctor and you want to spend more of your time making the animation on the actions strong instead of wasting time animating unneeded extras like face or tail. If part of the goal were to display talent at facial animation or versatility with quadruped to biped characters (it's not, the stipulation was biped- Quadruped is harder and takes twice as long which is why my professors usually vetoed quad or deformable chars when we started animating) then I'd say go for the ferret.
There's a difference between "seeking a challenge" and not being able to asses a job while knowing your limits and being unable to complete the task given. You can limit the character down to the very basics and still get a challenge out of it AND complete the task given- in this case, assuming the doctor, conveying a performance through a character that has no facial expressions, all physical performance and nuance. In actuality this is a far bigger animation challenge than with the ferret!