While an "Elvis Alive" bot can almost always be had on ebay for 0-400 "new", they do occasionally go for ~0 for used ones.
If you are not comfortable with the small form factor of the Pro mini, you can always use a costlier, relatively large and easy to wire up Uno (the Sparkfun spectrum shield is made to fit on an Uno).
Also, using a board with a USB connector makes programming much more convenient (no FTDI adapter needed).
Software: you will need to know how to program an Arduino at a very basic level. The Elvis is a very complex robotic head that has DC motors independently controlling head "roll", back and forth movements, eyes, upper lip, and the jaw.
The actual human being in the equation may or may not be aware of the situation, but it makes things that much ickier if they are — especially if the simulation in question is, arguably, not truly sentient in any meaningful sense (for an extreme example, remember that Pygmalion fell in love with a statue Aphrodite brought it to life).
If the inhuman entity is based on a real person, expect them to discover what's been going on at some point.
Compare Replacement Goldfish, Robosexual, Robo Ship, Robotic Spouse.
This probably won't actually work, but it's cool anyway.
Using "off the shelf" parts to make a life-sized human head that moves in a life-like manner and can be made to move its mouth in sync with sound, or even manually using a joystick.
Total hardware cost target: ~0You can also use what you learn here to apply to ANY other type of bot that uses similar dc motors to control movements (Wow Wee "Chimp Alive" for instance).