Thanks for the help, guys. It's nice not to have to answer every question.
You can use any coil-over that meets the specs in the manual. You can download a stripped down manual with the information
here.
The Jag subframe is an option because of the other rear suspension offered doesn't use the same one.
The street version usually includes carpetting, different seats, different upholstered dash with glovebox, and door panels. It may add more than $1500 to the basic kit.
If you put your own rear suspension together from a recycling yard core, I would certainly recommend rebuilding most of the components. They've been around for a long time. The differentials are pretty sturdy, but many of the XJs lack an LSD but it can be added later.