I wouldn't choose my roofers from new construction sites. In general, new construction roofing is horrible. Check out Angie's List. Google best roofing contractor your city and check out their online reviews at places such as Google Business. Don't necessarily go for the ones selling ad words, go down to see the ones that have an organic ranking. Pick 2 or 3, ask them for references from Customers they've done work for in your area in the past 6 months. Call the references. While the BBB isn't a great source, if someone has been horrible, you will see lots of complaints.
As far as ventilation goes, for a gable roof, shingle over ridge vent is probably the best choice. Go for the 4' plastic ridge vent instead of the roll ridge vent crap. The GAF Cobra Ridge Vent is a good one. I doubt many power vents are installed in MN and while power vents are a good form of ventilation, the failure rate makes them a poor choice if you have another alternative. Make sure the roofer installs it with 3" nails.
I'm not a fan of GAF Timberline shingles at all. Look at Atlas, I think they have a very good shingle at this time. 130 MPH wind rating. Ask your roofer to either put a gasket/rain cap on top of the pipe flashings or better yet, spend a little more money and get one of the lifetime pipe boots. Some are a bit ugly but pipe flashings are a major source of leaks.
Make sure Ice & water shield is installed in the valleys and eaves. With an 8 pitch in MN, I would guess two 3' rows would be sufficient.
Were it me, I would not install a roof window or skylight. The new Velux are much better than what they were 5 years ago but I still despise them. A tunnel is much easier on the roofer, easier to flash and less likely to leak. Even then, unless having it is critical for you, keep in mind you're putting a hole in your roof. Every time you penetrate the roof, you raise the chance for a leak.