Need WA Prime for K-01

I'm new to Pentax and need some reccomendations from experienced Pentax users.

I'm ready to pull the trigger on a K-01 + 40mm kit lens and I want to add a WA prime it can be 18mm , 20mm or 24mm or anything in between, it can be pentax brand or sigma tokina, etc, my only 2 criteria is that the prime be A-) Very Compact and B-) Cost under $350

Any suggestions?