Like Ddawg, I've done it on my own home also. I had THREE layers on. The previous owner said there were only 2. Anyway, that took longer than expected (more and longer nails).
You will definitely have repairs to do and maybe even have to re-sheeth the whole thing depending on the condition.
The newer architectural shingles actually make it a bit easier to install them. They are more forgiving than the old 3 tab system.
Here's where it gets tricky......yes, you can save money, especially on labor, but, remember, you will want to buy all the tools, etc to make the job go smoothly. Air compressor, roofing nailer, shovels to remove old shingles, hammers, etc. After doing mine, I vowed I would never do it again. It's time consuming (how valuable is your time?), difficult, expensive if you buy tools and don't have a way to dispose of the old shingles, and, what if you fall?
In the end, I would rather hire it out. Most roofing companies are done in a day or two, they're properly insured, they are much cleaner (covering the ground, picking up nails, etc.) and they do a better job than a one-man DIY operation.
Just my two cents'.