This is great for someone like myself who doesn't know a great deal about cars. Wheels and tires (the sizing, will it fit with X lift, etc.) are particularly mysterious, so pictures always help.
Can you comment on what sort of offset is generally regarded as better for offroading FJs and why? Intuitively it seems to me that having a negative offset would put more strain on the other components, but that's just a hunch.
This really depends on your suspension set-up and how far you want to push your fitment limits (how much trimming or modifications you are willing to do). Most of the guys here want to run the "biggest" tire they can with their stock suspension, leveling kit or lift.
If you have a lift (more than a mini lift or level) a wheel with a 4.5" backspace is generally the backspace that people go with for off-roading. (8" wide wheel with 0mm offset, 8.5" wide with -6mm or 9" wide with -12mm) With a decent lift most people want to run wider and taller tires than what came on the vehicle stock. To properly run an extra wide and
extra tall tire you need a wider wheel and you need the wheel to sit out away from the vehicle further than stock to avoid rubbing the suspension components and/or frame.
If you only have a leveling kit, mini lift or no lift at all you are more limited on how tall and wide of a tire you can run. In this situation most people choose to run a wheel with closer to stock specs so that they can keep the wheel and tire tucked under the vehicle in the deepest part of their fenders. (picture your fender cavity as bowl shaped) This allows them to run a taller tire but limits how wide they can go.
Will lower than stock offsets or low backspace increase strain on suspension and or other components? Its possible, but as far as I have seen, no one has been able to figure out to what extent.
...one thing I've always wondered about is space needed due to the brake caliper on our FJ's. BellyDoc explains it as 'X factor'
. Is that a normal measurement that you can just go look up when looking at different wheels?
I wish there was. Unfortunately "x Factor" is really only used by wheel manufacturers. It would be nice if there was a simple 1,2,3,4 etc scale of something for brake clearance.