So it sounds like you have decided what you're going to do, right?

I have had lots of friends and family come to Colorado to visit me from Wisconsin and I've seen a variety of responses to elevation as well. It seems like physical fitness level and water intake are a big part of whether or not you get sick. I say drink as much water as you can, and if you are in good shape you should be fine, especially with your plan to move up to higher elevations slowly over the course of your trip.

If you are still not sure about your plans, I have to always recommend the Black Canyon of the Gunnison. I love Colorado and I know there is a lot of great stuff to see on both sides of the divide, but Black Canyon is on the West side. It's a deep canyon though, so it's not at the higher elevation of the mountains and may be free of snow. It's magical to see, but climbing down into the canyon in October probably won't be safe unless there really is no snow.
Rocky Mountain NP is awesome and has lots of great hikes to offer. Even in the dead of winter there are accessible trails and lots to see, so I think you'd be fine there in October.
Just wanted to suggest that you look at lower elevation ideas like Black Canyon and maybe Dinosaur national monument, Colorado national monument, etc. None of those are exactly at sea level, but they may be less snowy than RMNP and other higher elevation options.