Well, let me start with what I know about...a 4 on 12 pitch is plenty for a shingle roof, especially on a garage. Metal is unnecessary and will cost more, and it sounds like you need the money for your waterproofing.
Hydraulic cement is fine for the backside of the foundation wall, but while you're at it, why not install a 6-mil polyethylene film right down and over the top of the footing? Very cheap insurance. And yes, it's best to hand excavate.
If the sewer under the slab is cast iron, you have very little to worry about. And cast iron pipes CAN be cleaned, if there is a lot of scale and stuff built up in them.
Keep us posted, and good luck.
Gutters are always a good idea. Make sure when you install them that the top outer edge of the gutter is below the slope of your roof, projected out over the gutter. For low-slope roofs, the distance should be an inch. For high-slope roofs, a quarter-inch is adequate. Medium-slope roofs...somewhere in between. If you don't, you may find your gutters either hanging off the house or on the ground a day or two after a heavy snowfall.
Gutters are far better than letting the roof water splash below, no matter what it splashes on.
Architect (NY) and Home Designer (PA)