IMO, what to expect all depends on where and when you go. If you go to a guided town in S.D. in June the dogs will most likely be very skittish and after the first shot 200yd's to 350yd's will be the average, if your lucky. On the other hand, if you go to N.D. in April, the dogs are a lot less educated. You can shoot a long time at 200 before having to move out to 300 and beyond.
I always say "average shot is 200yds", be able to hit a 20oz soda bottle (pretty close to the size of an average p.d.) at that distance.
If your not shooting from some sort of bench, I recommend knee and elbow pads. Something to put under your torso is good too. I've went prone more than once onto a cactus, that sucks.
I've never had a bug bite me in the Dakota's, but I guess it's not a bad idea to bring the spray.
+1 to lots of water, lots...
Varmint reticle is cool, but not a guarantee. It's biggest benefit is the point of reference it gives you. I won't spend the money on one, but have used others.
3-9 is OK, I use a 6.5-20. Like I said before, look at a 20oz soda bottle at 20yds with it at 9x, you might want to move up.
Bringing one gun is fine if you take your time and do some spotting. Going slow with 1 gun you will go through between 300 and 600 rounds if it's slow (weekend). With one gun I wouldn't bring less than 500 rounds for a weekend, no more than 1000.
In my opinion, the most important thing to do to get the most shooting is to set up downwind. This doesn't sound like a big deal but through a lot of trips and trial & error I am 100% positive it make a huge difference. You can almost always count on a 5 to 15mph wind in the prairie. Shooting into the wind does two things for you. First, it pretty much takes windage out of the equation, not completely, but much better than shooting in a crosswind. Second, and most important, the incoming wind does a great deal to deaden your rifle report at range. If your a hunter, you might relate to being able to hear gunshots in the distance better when your downwind of them. The dogs will remain above ground longer, and since you can't shoot them when they go down, it is the most important factor.
Good luck and let me know if you need more...