Welcome to Colorado!!
I completely agree that the traildamage.com website is the best one out there. Definitely check that out. As far as some starter suggestions, try the Johnny Park road SE of Estes Park, and Pierson Park road in the same area. Up the Poudre river you have the Old Flowers roads which are pretty mellow, and Kelly Flats/Heart Attack hill which will give your machine a good test.
Further south you can do the east side of the Moffat Road, an old railroad grade which is extremely historic and scenic, but easy. In the same area is Kingston Peak, which is more challenging and has great scenery.
Further south off of I-70 is Loch Lomond and Bill Moore lakes.
Finally, a really stunning area is up Montezuma road behind Keystone. Pick a good day because you'll spend most of it above timberline with 100 mile views if the weather is clear.
Slee Offroad in Golden is the primo place for mods. They are good but not cheap. Visit their website - they put all their standard mods out there with prices, and also have some great tech articles. Addicted Offroad in Ft. Collins gets mixed reviews. He knows what he is doing but getting a schedule out of him appears to be impossible. I've heard positive things about Bullhide Offroad, but have no first hand knowledge.
I'm in Loveland, so pretty much right next door.
Have fun!!