Most commonAllen wrench sizes

I would just buy a kit that has a wide variety. I also recommend a set of 6” Tee handle wrenches. 
Many metric wrenches aren’t just Rotax since many planes come from Europe or some other country that uses metric on the entire plane. If it’s US aircraft like an RV12 you will need both imperial (standard US) and metric. The kits aren’t expensive and can be purchased at any hardware store or Amazon.