If you are coming from Spit V & VIII, the easy way to get into the 109 is to start with the F. It's reasonable close in performance and a true turnfighter. That way you can get used to some of the originalities all 109's share like the wing slats, the centerline armament and the stiffening controls at high speeds.
If you are comfortable start to work your way upward: G-2, G-6, G-14, K-4, experiencing a mighty increase in engine power while suffering from worsening turn ability and handling.