The wider you go, the harder it is to find off-the-shelf rims that will fit because of clearance issues. The backspace is the distance from the pad (the center of the rim where the bolt holes are) to the back of the rim edge. The maximum allowed there is going to depend on your suspension and how much distance is available before the rim contacts the strut. I have a 6" backspace on my 9.5" rims and they clear fine both front and back.
The offset depends on the style of wheel and how much movement in or out is needed for the spokes to clear the brake calipers. My rims have a 28mm offset for example. The stock rims have a 35mm offset. The spokes on my rims have better clearance than stock so they could tuck in a little more which allowed them to fit without clearance issues on the fender lip.
The first critical measurement is clearing the brakes; once you know what that distance is, you can check the other dimensions for clearance. It will depend on the spokes of the style of wheel you choose as to what that measurement will be. Measure from the base plate of the hub where the bolt studs are, to the top of the brake caliper and add a few millimeters for clearance. That's the distance you need from the base of the rim pad to the edge of the spoke where it passes over the brake caliper.
Hope all that makes sense!
