It is confusing but this is what Discount Tire says.
Discount Tire
Wheel Offsets
The offset of a wheel is what locates the tire and wheel assembly in relation to the suspension. More specifically, it is the measured distance between the hub mounting surface and the center line of the rim...
Probably not. It's super easy though. All the tools are in the car. Just add masking tape and high temperature spray paint. The time is in the prep with the tape. Took me 2 hours. A tire shop might do it for you......
I guess it depends on your environment. In Arizona, the roadways are "wheel friendly". Not many potholes and newer roadways so 19's are fine. I would run 18 in cold weather states with rough roads with potholes....
I compared the height and width of the factory wheel and tire (215 50 R17) with my new wheel and tire (235 35 R19). Significant difference in width but hardly any difference on height. Lose 20% in noise and ride quality. The new wheel is 8.5 with a 45 offset.