Done the one-man auger thing. Hope you're a "hefty" guy.

Works OK, unless you're in roots or rocks. You'll have bruised ribs...
430' is only 50+ holes, not that big a job...DEPENDING ON SOIL CONDITION (disclaimer). There was a time I would have tackled that with a rental or even clamshells, but now I'd rather be on a tractor seat. If you rent, you can save some time by marking holes ahead of time with a bag of lime and a dixie cup.
1. I use 8' on-center. Looks good w/rest of fence. Just have to fold in pickup mirrors. The wider the gate, the more sag. Split rail gates are kind of a pain to build. Need good diagonal bracing and hard to attach hardware.
2. I think 3-rail is nominal 48" to 54" at the top of the rail. I put mine about 3' in ground, depends on how much space you want between bottom rail and ground.
3. Whatever looks good to you.
Here's another split-rail "trick." Once you decide how much post is to be out of ground, take measurement (top down) and chalk/crayon post at ground line. Measure/mark all posts. Drill all holes slightly deeper than length of posts below mark. Drop posts in holes, assemble rails, go back and kick dirt into each hole to bring post up to ground line. Then fill/tamp each hole, and you're done...don't even need a level, plumb the fence by eye.