Well in your climate, if you're replacing a simple low-pitch roof, it shouldn't cost too much. Even the worst of asphalt shingles (15-20 year 3-tab) should last 15-20 years there. So a bigger question would be why do you want to replace the roof so soon? Why take something at half or less of its service life and replace it? Not very cost effective, even if you don't like the current one.
I've only done asphalt roofs. If you're interested in doing your own roof, Family Handyman magazine is a good place to start. You're going to need help, even in simple roofs. Unless you're trying to be extra manly, if your lot is situated for it, get the shingles boomed to the roof and spread them out. I don't care what anyone else says - never do an overlay. You won't be able to address/evaulate any underlying structural issues (rotten sheathing) and the new shingles will lose at least 5 years of their life immediately. Take the time to strip properly.
Talk to building inspectors in your area - they can give you an a better idea of the code for ice & water shield for your locale, as well as the price per square going rate (I hesitate to generalize because this is a huge country with a lot of variation). Time depends on your size of crew and difficultly (pitch & height in your case) and what tools you're using (compressors w/nail gun or elbow grease/hammer). If you are going the manly route, the elbow grease & hammer is better (less chance of shingles blowing off because of misplaced, over/undershot nails, etc.), but you're be unlikely to find many roofers doing it that way nowadays (there are old-school hard-core guys in my area that still do).
