I'm confused. Didn't the OP say he was looking to make "flat panel doors" with mdf centers and go cheaper than a door bit set? Poplar would be a good choice for r&s frames, but for tooling flat panel doors... which are along the lines of a Shaker style door.
I'm thinking easiest for him to create a flat panel door would be a Tongue & Groove bit set. Cheapest would be to create the same joints with slot cutters.
"Don't worry, I saw this work in a cartoon once."
"Usually learning skills and tooling involves a progression of logical steps."