looking new wheels but Back spacing and offset ??

I am trying to figure out what back spacing to use on my wagon, I want to run and 18" wheel. I don't seem to under stand the need for the offset when the back spacing should be all you need. Can some let me know if either of these would work 18"x9.5" with a 5.5" backspace or an 18"x8" with 5" back spacing and where or why would an offset measurement come into play?

Backspacing is used to determine the inside clearance to hitting stuff like wheelhouses and is pretty commonly the biggest issue in going bigger on tires.

But, IMO, offset is the most important in relation to handling and steering geometry. I think the stock offset is +8mm, but the parts they used may actually be old ones that were designed for zero offset. Changes in offset of up to maybe .5" don't seem to be majorly noticeable in driving, but more can be depending on your pickiness and feel.

Be aware that offset, backspace, and wheel width are all interconnected so you can't change only on of them without changing at least on of the others.

Best to just search explanations for the wheel dimensions and see the illustrations, if you aren't familiar with how the interconnect.

Stock offset is +6 mm. Offset and backspace are functions of each other. Offset is the distance of the mounting surface from the wheel's centerline, and backspace is the distance from the inside of the rim to the mounting surface. If you add half of the width of the rim and the offset, you get the backspace.

The rim is designed to fit with the OEM offset and stock rim width. If you get a wider rim, it can contact the suspension, and the tire can contact the body/frame. If you change the offset, the rim may be closer to the inside or outside of the car. The tire size on the rim will also affect the clearances. A larger rim may cause different issues, depending on the width, offset, and tire size.

You can check the fitment here:
http://www.rimsntires.com/specspro.jsp

Enter the stock tire/wheel size with a 6 mm offset in the first block, and your pick in the second. There is an analysis block below the second block that will give you information as to the possible interference you may encounter.

18x8 with a 5.5" BS will work just fine.