Gutters are designed to collect rain water and place it away from the foundation. They also keep water from rotting doors and windows.
Gutters are typically priced by the lineal foot. Gutter removal, corners, downspouts etc... will likely be priced ale carte. 5" gutters are common for Michigan. More and more of our home builders are using the larger 6". Soon, 6" will become the standard. I put 6" on my family's homes
Most gutters are aluminum and will not rust. They should last for decades if kept clean but we recommend you invest in a set when you replace the roof. Michigan weather and temperature changes are brutal to the hangers and caulk. Also, deforming the metal
If your neighbor does...so should you. What I mean to say is, most neighborhoods are built around the year. Aging is near the same.
If your gutters are leaking, pulling away from the house or installed with the outdated Spike and ferrule system. We use a very durable hidden hanger that is built to last.
If the wood is rotted, it should be replaced. I always recommend adding a aluminum fascia to weather proof the lumber
Leaders are more typically known as downspouts. The attach to the gutter and channel the water to the ground. 3x4 rectangular are standard with our installs, not the smaller, restrictive 2x3.
The new "High Volume Miter" handles water from any size valley with out splash over. It also allows the water to move very efficiently through the gutters.