There is a lot of potential here.
Don't buy finished wood from a BBS. Wander out into the country a bit and find a saw mill and buy seasoned rough cut wood. For instance, I recently bought about 20 2x 8-10" 8' rough cut maple for less than $100. That would make several bookcases without a bit of ply or buy about 2 1x10 4ss boards from a BBS.
Your area should influence the choice of wood for the bookcase (you say it is not intended to be an heirloom so specifying a wood type is much less an issue). I use what's plentiful, cheap enough, and meets aesthetic and workability demands. For me that boils down to Walnut, cherry, oak and a few other less commonly milled species.
I like the bookcase design, but I would change the curved sides to slightly angled (7-10 degrees). Leave the arch sill under the top and in the kick plate. The arch will stand out more and could even be increased in height a bit.
If you don't have mill capabilities (planer, jointer, accurate table saw, band saw) then read and learn to mill lumber by hand. The size of this project would not increase the time invested a whole lot (but would increase the cost in energy input significantly, well calories versus amps) and with 3 planes, 2-4 saws,and a few chisels everything could be done by hand.
I have a few early American antiques that have 10-14" wide boards in them that have not warped in 150 years or so (and I have others that have warped). They are all quarter sawn. Finding seasoned, quarter sawn hardwood today to complete a project will put the investment in the heirloom category.
Learn to joint. Stickley did.
If you use panels, raised/flat or otherwise, you need to stain/finish/paint before assembly. Wood moves.
Dave.