I use 1/2" prefinished birch for the carcase.I leave it natural inside the cabinet.On the side,use 1/4" ply if it's stain or 1/4" mdf if you are painting it.Glue the 1/4" panel to the side of the cabinet.You only need it where it shows.If you want to paint it inside too,then you can use 3/4" MDF for the sides and then you can use 1/4" for the backs depending on how you are building your cabinets.
As far as the face frame,use soft maple for paint jobs. Poplar is ok,but stay away from clear white pine, too soft.
If you decide to use MDF,you can use a oil base primer,or if you can get it,i like to use a vinyl primer.Then i use a precat lacquer or CV as my color top coat.