Whatever you buy, you should try to make sure the rolling radious of the new wheels/tyres matches that of your old setup. So whilst the above post gives an indication of the usual tyre sizes on said widths, it doesnt take into account what your current rolling radius is.
Google search for a tyre size calculator and figure out from there how to work out your RR.
As for offsets, yes if you take your existing wheels off they might have the offset stamped on the inside of them. Note front and rear offsets could be different.
To work out your offsets, i believe it's the center point of the wheel width (so 4.25" in your new wheels case and then subtract 45mm from that for your +45 offset.) You then need to work out if that will leave you as much space as your existing wheels when mounted to the hub.
See graphic below to try to explain.
http://kgm.tiwing.com/calcs/img/offsetdiagram1.jpg