I acctually wasn't entirely positive that companies don't go, but we looked at img, rim , and alpine accents among others
There are both many really good and some bad guide companies in Ecuador. Check recommendations with books (or forums) such as Rough Guides, Lonely Planet, or Moon publications. Plenty of people on Summitpost have used guides (use the search function on this site) in Ecuador and have good recommendations.
I'd suggest using an Ecuador based company with a good reputation. This will probably cost 1/3 that it will with a western company and they use the same food, huts, vehicles, routes, etc. Plus you can more or less choose your own mountains and schedule.
Most of the Ecuador volcanoes are fairly straightforward ascents (similar to Mount Rainier, but with high altitude), but there are objective dangers such as avalanches and crevasses and bad weather is common. If you aren't experienced or are fairly new to the sport, it's definitely recommended that you go with a reputable guide. There are some good mountaineering schools there as well (sadly, Moggely is out of business, but there are some more good ones).
Outside the Ecuador volcanoes, some of the mountains in Peru or Bolivia should provide good training for Denali.
As far as Elbrus goes, it's about the same difficulty as Rainier, but with less crevasse danger as long as you stay on route. It does have a taste of high altitude and is generally colder than Rainier. In good weather, it's straight forward, but the weather can be atrocious, especially outside the July-August season.
Of course off season ascents of the Cascade volcanoes are often used as a training ground for Denali. Until you have the proper experience, it is definitely recommended that you take a guide or enroll in a mountaineering class for any of the ascents above. Taking one of the week long seminars in the US or in places like Ecuador are certainly a good way to train and learn the basics. After learning the proper techniques, and after several seasons of practice, you can set off on your own.
Mountains such as Cotopaxi, Elbrus, Blanc, etc. certainly do attract a lot of people that either underestimate the peaks or don't have the proper skills. Unless you have some kind hearted mentors who will train you for free, it's best to learn the proper way first, even if you have to pay for it, before going our on your own. There is no shame in using a guide or taking a seminar or classes to learn proper mountaineering techniques before climbing independently.
19 is a good time to start preparing.
Good luck.