As said, those three places have excellent reputations and are tough to get on with. Most of the well regarded operations in Colorado (especially the smaller ones) give heavy preference during a full time hiring process to their part time staff. The service I work for (very small, 15 FT employees) didn't even consider outside applicants for our last opening.
I was able to get that spot by being an extremely dedicated part time employee, which as a young person with little out of work responsibilities, wasn't a big deal. Could be different for you of course. I had a fulltime job with a bigger ambulance company for most of the time I was a part timer so I was working a ton between the two jobs and often struggled with my commitment to being full time one place with a desire to be full time elsewhere. It was worth it in the end though, but I'm not sure it was sustainable. I couldn't have operated like that for much longer I don't think, so maybe I just got lucky.
As for those agencies, I know Summit has had some serious financial issues as they don't receive very much money from the county and have to rely primarily on reimbursement. They have seasonal units for the winter but have to cut them in years past due to a decline in skier visits though I think they reinstated them.
Eagle County recently merged with Western Eagle County to form Eagle County Paramedics. They cover the county, but really aren't that big and also do seasonal units I believe.
I don't know much about Thompson since it's not really an area I have much desire to live in right now, but they have a great reputation and are much bigger than the other two.
Cost of living in Denver is far mroe reasonable (you might even call it cheap) compared to the mountains, Colorado Springs is even cheaper. Boulder is rather pricy but most of the other Denver sub/exurbs are reasonable.
