I recommend :
1) leave your 22 rifle at home. If they are within 100 yards you can hit them with a 22 pistol or the 223.
2) Take your 6.5 for the distance shots.
3) buy a Rem 700 223 Varmint for the rest of them. Use whatever highest power scope that you have. No matter what you do in the future, having an accurate 223 bolt gun is fun.
4) buy a cheap wind meter and ballistics app for your smartphone
DO NOT make a huge investment, because after you have tried to shoot prairie dogs in the North Dakota winds, you will probably decide on something else for next year.
I have been shooting p dogs for over 50 years, starting when we antelope hunted in Wyoming, and shot pdogs while we waited for our antelope to be butchered and frozen. We found out pdogs were more fun than antelope.
I used my Sako 243 in those days, but switched to a Rem 700 Varmint in 223 when I just hunted dogs (now working on my third barrel). Tried a heavy barrel AR in 223 (magazine projection out the bottom is inconvenient on rotary table), then I went to a 22-250 (because of the wind) and am working on it's third barrel and it is my back-up gun, then I went to a 6BR single shot custom gun (because of the wind). I now shoot the 6BR with an 88 gr Berger Varmint (high ballistic coefficient), which is like a Berger 105 hybrid without the boat tail, and I am working on my third barrel.
P dogs will not let you shoot the area very long before they duck back into their holes, out to about 400 yards or so, so you either have to move, or wait them out, or just shoot the long shots. Since we shoot off rotary tables, we don't move very often, so I have chosen to invest in stuff that will help me in the long shots in the wind. A good rotary table, rangefinding binoculars, a good front and rear rest, wind and atmospheric measuring equipment and a good ballistics program. Also a supply of cold beverages while I am waiting for them to show their faces. For reference, our regular group of 4 shooters use a 6BR, a 22-250, and two 204 Rugers.
I also recommend you consider the binoculars, rangefinder, table & rests (or bipod and rear bag to shoot off the hood of the truck, we used a tarp on the hood, and 4 bungee cords to fasten it down), a wind meter, and a ballistics app for your smartphone. You do not need all of this, the binoculars are probably the most important item, rangefinder the second. Again, do not spend big the first trip, you will probably change your mind about what you want next year.