Sorry, can't answer questions until we've seen pics, ;-)
The XJ6, 12 and S have a completely different style control arm. They are asymetric on E-types versus an isosceles triangle on XJ's. The spindle looks very similar and may be interchangeable for some bigger brakes and vented rotor options. I put some large by huge front brakes on a CCX with 17" rims and was completely amazed the rotor/caliper size that almost fit in a 15" wheel. E type brakes are lousy especially with wrong ratios on pedals with no boost etc. I wouldn't worry about rear brake upgrades unless you're planning on tracking the car.
Here is an awesome bolt on brake upgrade
Wilwood Racing Caliper Conversion Kits
I really like the Wilwood upgrade options and use them on my own and customer cars where I can. I have the caliper brackets to put Wilwoods on the solid rotor Jag brakes.
Are you going with 6 pin hubs/wheels? If you use 5 they can go right on your Jag hubs, same as a Chevy 5 on 4.75" B.C.