You should be prepared for anything with regards to the weather and snow and freezing temps are quite possible, especially in Colorado. All the drives between your destination points are doable and comfortable enough to be able to take your time if the conditions demanded so. You have options on your routes and if the weather was kind I would travel from Keystone to Durango via US50 and US550, aka The Million dollar Highway. I would also be tempted to stop one night on the way and have one less night in Durango. You will go over Monarch Pass, through Currecanti National Recreation area and be near to Black canyon on US50. The Million dollar highway is spectacular and the mountain towns of Ouray and Silverton are lovely. There are high passes along the way and you would have to check road and weather conditions. I would imagine you have planned a visit to Mesa Verde NP from Durango. You could also consider one day/night at Bryce canyon between Page and Zion NP, but that is at high elevation and can get extremely cold.