Welcome to iboats...
As a guy who lives and boats in Colorado, I too boat at elevation (4,300' and up). The boat/engine combo you are asking about is going to be under powered for that boat @ elevation. I would say a 90hp or larger would get you in the ideal-perfect performance range for that boat.
What is your home lake?
We frequent Lake Pueblo, Horsetooth, Lake Meredith. All but Meredith have a fair amount of traffic to include large 30'+ boats which means choppy water on the weekends and in a smaller boat, it's a less than enjoyable ride....oh yeah, don't forget the wind!
I have towed 2 skiers at a time, wakeboards, tubes and large inflatable towables...I have almost 3 times as much hp.
I would recommend nothing smaller than 18'-20'. My boat is 18'6" and I wish I had 2 more feet (two-footitis!)
Anyway, as most would say, It's your money and you should purchase what you want, but the mere fact that you're on here [iboats] asking for advice at least means you have some reservations. You will get great advice, whether you heed it or not is up to you!
Is this it?
http://denver.craigslist.org/boa/2860146629.html
Happy Boat Hunting!
Brett