I was living at sea level before moving to Denver and didn't experience any altitude problems in the city. It only became an issue when going to the mountains which can be at elvations of 9,000ft or higher. Once you acclimated for a few weeks, you should be fine, even at higher altitudes. One thing to consider when in Denver is your alcohol tolerance will typically decrease.
In regards to sun and skin issues, you definitely have to be cognizant. Compared to the Midwest or East Coast of the U.S, Denver has far less trees. Its not barren and there are plenty of tree lined streets, buy I would always have a hat and sunscreen ready as it is easier to burn.
In regards to employment, I can't talk specifically to that, but I do there is a large tech and military presence in Denver/Colorado Springs.