Ogres in the Distance

My players are travelling from Sandpoint to Magnimar and I would like to through a semi-random encounter at them. They are 6th level, 5 players with 25 point buy, so they end up about apl 7.5. I'm thinking a group of 4 ogres including 1 spellcaster that have taken some slaves. I also want this encounter to start with the PCs having ample time to decide how to handle it, so on a relatively open stretch of plain. This leads to 2 questions.

1) what spellcasting class is best for the ogre? The +4 for a classed npc will bring its wisdom up to a 14, while Int or Cha max at 10 and 11. This seems like cleric or druid would be best, though empyreal blood sorcerer could work. Finally, I could just use the scarred witch despite it not being an orc. I figure 4 levels of spellcaster.

2) encounter distance? According to the corebook, the Max encounter distance in a plain is 1400 feet. The best perception check, including penalties for size, allows the ranger to see them at 400, which seems a little close. I was thinking that he could possibly see a couple hundred more feet, but would only be able to identify that there was something moving. At 500 feet or so, he can identify that there are large and medium creatures in a group.

I apologize for any typos. I'm typing on a kindle fire which is pretty bad for autocorrect.

Concerning 1, have you thought about the Scarred Witchdoctor? Seems perfect for an ogre.

Introduce your players to the ACG and have the spellcaster be a Shaman.

Is the Ogre Magi no longer a creature? I couldn't find it in the Bestiary.

http://www.d20pfsrd.com/bestiary/monster-listings/outsiders/oni/oni-ogre-ma ge

you mean this guy?

probably not really a caster enemy. I agree with Kalridan scarred witchdoctor seems just about right.

Is the Ogre Magi no longer a creature? I couldn't find it in the Bestiary.

Not related to actual Ogres in any way.

Never even hang out together.

Pathfinder Ogres are cruel inbred rapists, who play childlike games with the corpses of their victims.

They are the grossly oversized versions of the hillbillies in Deliverance, with more of tendency for violence, and a greater insatiable lust for the violation of others, living, or dead.

I'd say the ogres can be automatically spotted across the plain if they aren't bothering to be stealthy. The -140 to the perception check for distance only kicks in if the ogres are bothering to hide; otherwise there's no opposed role to be made.

It'd be reasonable to require the party to close some distance to realize they're seeing ogres with a slave train, though - they're looking at the ogres from four football fields away. Could be hard to make out details at that distance.

Hmmm. Even if you allow the ogres to use an orc witch archetype, they still needs Int (specifically, to pass a DC 16 or 17 spellcraft check) to actually learn spells beyond the free ones that come from leveling. Though if the ogre's just for a single fight, it's fine if the ogre only knows a smattering of spells.

And yeah, the more I think about it the more I'm liking scarred witch doctor for the caster ogre. An ogre getting to use Con for its save DCs is NASTY. Putting the ogre's other +4 stat adjustment into Int gets to where it doesn't have a penalty anymore, which turn gets around the spellcraft issue I brought up. Nice.

Scarred witch doctor is sort of what I was thinking. Change the stats to; Str 19, Dex 10, Con 20, Int 10, Wis 12, Cha 7. I'll grab a few spells. Any suggestions on hexes?

Insofar as perception checks go, I would think some check would be called for to see when you become aware of the other. Even if they aren't hiding, the PCs won't just be staring directly at that spot on the horizon.