Good questions all, and hard to answer.
You might start with the Colorado Outfitters Association. (do a search)
A lot of times you can get a "read" on an outfitter by looking at their operation. The Association might have certain criteria which weeds out the duds. A reputable outfitter should also offer a list of "testimonials". Good research, but read between the lines.
Keep one thing in mind, an outfitter cannot guarantee you taking a bull, any bull! But they should be able to guarantee putting you on them. Even a drop camp. I mean, they aren't gonna leave you in the parking lot of the local mall! Never know though, a lot of mountain communities get wildlife visits. I live in "Cheyoming" and we've had Moose, Elk, and even a black bear got treed at one of the golf courses. Talk about a hazzard?!
But I digress . . . Find an in state outfitter that can do all you want. Price, drop camp, Big Elk (or Any!) and whatever else you need. Find out if they are the actual outfitter, or just an agent.
Elk aren't whitetails. They live at altitudes that can make you sick. They can travel 5 miles where you go just 500 yards. They can travel miles after being shot in the boiler room. They might make it look easy on TV, but get in shape, and bring a big enough gun (That you are very familiar with and confident with) that will bring down an animal the size of a horse.
AND! Understand that once you've harvested your animal, then the work really begins.
Hope you have a good time. There's nothing else like being in the Rockies in the Fall.