You have a situation similar to one I had when I bought a new rifle a few years ago. New rifle, new scope, no favorite reload or factory load that's known to be good. Worked up a few loads that shot decent, but wanted better, but the more I shot, the worse the groups were. Tried everything I could think of and it still shot terrible. I went through hundreds of rounds. Finally, due to a magazine article I changed out the brand new scope for an old reliable one. All along, the scope had been the problem. A brand new scope. Oh, well.
So, if you still have the old scope on it, swap it out with new or borrowed. If you go with new, the options are endless, and most are good. Go with brand name gear. Bushnell, Burris Fullfield II, Leupold VX2, Vortex will do just fine. Magnification range of 4-12 will be just dandy for seeing the bullet holes at 100. Don't listen to the baloney about sighting it in at 4 power. Why do that if you have 10 or 12 power available? As for mounts, the DNZ one piece are fine, and easy to install.
As for bullet choices, if you don't reload, Remington, Winchester, Federal ammo will do just fine. Your rifle might like one more than another, so try a few out. Bullet weight of 150 or 165 will be fine.
As for bore cleaning, lately I'm in the camp of not taking all the copper out till accuracy degrades. That's 25 rounds in my 220, about 50 to 75 rounds in the 223, and seemingly never in the new 260 with the Brux barrel. Until you start reloading and have constantly consistent ammo, you may not be able to really tell when accuracy has degraded and copper needs to come out.