I was planing on something like this for the Arduino power
Buy stock in the battery manufacturer, first.
Also I think the 2AA battery step up wouldn't be additional hardware if I get this
additional hardware... OK.
I assume the 4AA pack that I would need is this one
That will work.
Does everything else look good?
Yes.
Any advice on how easy/difficult this will be to set up?
Getting the hardware connected is easy. Configuring the XBees is easy. Getting them to read and send the accelerometer data will be the hardest part. Writing the code to receive the data from the remote XBee, and make sense of it will be the most challenging part of the project.