When it comes to big purchases i say to myself "Can i afford it? Are the bills paid? is my daughter happy? and do i deserve a treat?"
If i can answer yes to all of the above then i'll go out and get myself whatever it is (last purchase was my D60)
I think though, that simply by virtue of the fact you have posted this and are in a quandry over it, you should hold off the purchase for now. If you're seeking a justification then the purchase will simply be riddled with guilt.
JMO