Then why didn't you do a search and educate yourself by reading through all the info already out there?P.S. I know this question has been beat to death... Sorry
If it wasn't for the mudding height requirement, it would honestly be an easy answer in my opinion: Old Man Emu. Unfortunately they don't go nearly that high. If you're lifting that high, you have a lot of research and planning you should do. Lifting 4+ inches is not something you just buy a simple kit and install for. You'll have to do something with your track bar, your control arms, SYE, diff pinion adjustment...if you're going to put big tires under that lift, you might want or need to re-gear and recalibrate your speedo (very quick and easy)
Since OME isn't really an option for you, then I'd personally piece my own lift together rather than buying a kit. If that's not an option for you, then I'd probably go with BDS or RE.
P.S. First thing's first indeed. Hopefully you've done or plan to do some thorough maintenance on it; I've never seen a used vehicle for sale that wasn't neglected in some way. It's wise to just change all the fluids and do a tune-up when buying a used XJ right away--it's extremely cheap insurance.