A new Leader 722ts Build help?

The rake on it though is 30mm. Now i've never ridden with one of those before, i understand that kind of rake is aggressive and gives a twitchy and very responsive steering

Ah, that old wives' tale again. If you use a fork with less rake, you actually get slower steering due to the increased trail. However, short of riding otherwise identical bikes back to back, you're unlikely to notice the difference between 30mm and 43mm.

21cm steerer enough for 58cm frame?

I doubt it. Measure your head tube and add 75mm for a conventional headset or 45mm for integrated headsets to get the practical minimum. Any less than that and you'll be back next week quizzing us about low stack headsets and stems.

crank arms. 165mm or 170mm? I've never ridden 165mm. is the difference in power transfer really that noticeable for a non competing rider? From searching on Google and around here that seems to be the case. Just wanted to double check.

Nobody has ever proven a performance difference, and most of the people who claim they can feel a difference are kidding themselves and are proven to be deluded if you swap their cranks without telling them. Shorter cranks increase your gain ratio, all else being equal, so you should reduce your gear ratio and increase you cadence to compensate if you really want to compare like with like.