Nice job. Yes, moderately fast character speed with extra spacing is a very good way to go as you'll be less tempted to count dits and dahhs and will just start hearing the patterns. There's nothing magical about 15 wpm for the character speed so if you needed to drop that a tad it would be fine and if it all starts making sense you could increase the character speed a bit and then over time increase the overall word speed. There's nothing wrong with the MFJ-550, it's a decent working key but if you look around you may find something nicer with a bit better construction for a similar price. Something like an older used military key is often a good way to go. That said, the MFJ-550 will definitely get the job done.