Here's my take/opinions - keep in mind they're just that, opinions get a few different ones and see what works best for you.
1. I think the Spit is a great starter bird for someone who's interested in air-to-air fighting. It's reasonably fast, climbs well, accelerates well, has a good turn radius and the 20mm have plenty of punch. That being said I think the Spit XIV is probably not the best of the bunch for a new player, I would tend to recommend either the Spit VIII, IX, or XVI. The XIV is probably the least forgiving from a handling point of view.
2. As for convergence, within the effective range of your weapons, it is largely pilot preference. For either the 20mm hispano cannon or the .50 cal MG's 300 yards will give you plenty of hitting power, the only advice I can offer there is there isn't one magic convergence that works for everyone. What's far more important is knowing the drop and lead at the convergence you're using. Aim is far more important than convergence so don't change it too often in search for the sweet spot.