Hi Olivetree,
There's nothing wrong with pine, if you buy first quality joinery grade. A good timber merchant will be able to advise here.
If you buy it ready planed you take the risk that it might not have been planed carefully.
I have employed Jacob's strategy, if I can get the timber for the right price. Quite often a badly bowed piece of timber in the racks, has moved as much as it's likely too, and with a good machine you can end up with serviceable sections.
What I thought most about here is the span of the shelves. But I am quite sure you wouldn't neglect to use intermediate upright supports.
So, as your customer isn't stipulating anything in particular, I would use good quality pine, for appearance. No fiddling around lipping birch ply; which wouldn't be much (if any) less expensive.
John
