I recently upgraded my grandmother (85) from a very sluggish desktop to an Asus Eee Pad Transformer.
A tablet was the best choice because the touch interface is more intuitive, the device is very portable, and there aren't many updates to do to it (antivirus and antimalware was always a concern with the desktop).
Most of the tablet fit into the price range I was looking in. Using CNET's review I narrowed it down to the Sony S, Samsung 10.1 Tab, Asus Transformer, and the iPad.
While I am a huge Apple fanboy, and prefer the simplicity of the iOS interface, I felt that the iPad was too dependant on a computer with iTunes. This was mainly due to my personal situation; it may be different for you.
So we bought her the Transformer with the keyboard dock. The idea was that, having used the desktop, she was more familiar with typing on a physical keyboard.
Right now, she is still learning how to use the device. She is struggling to get her email to send (a lot of them are staying as drafts... still working on this problem), but enjoys using the browser. Typing is difficult without the keyboard, and she has trouble navigating back to correct errors. Besides the browser (Googling) and email, she doesn't use many apps.
She has it for about two months now, and the learning curve is identical to what it was with the desktop. However, when she starts to understand the device better, I am confident she will like it better than the desktop.
My advice for you would be to narrow the number of device down, then if possible take you dad to the store to hold and feel each device. After that, as he starts to use it, gradually introduce him to more and more features. I made the mistake of showing my grandmother everything the tablet could do (taking pictures/videos, downloading apps, deleting apps, adding widgets, changing the wallpaper, moving icons, etc.). Stick to the basics: opening/closing apps, using the keyboard, moving the cursor to correct errors, etc.
One final warning would be to avoid the super cheap tablet. I was very tempted by their price, and they seemed to have just the features my grandmother needed, but the quality and support wasn't there.