Mark I would recommend going to the library and looking at a Chilton's manual for an XJ6. In particular, the rear suspension, HVAC and twin tank fuel system. If you want something like an M5 level of performance, fast, comfortable, capable, quiet, then I would plan on spending on the order of what an older E39 M5 would cost, for hardware, plus of course the labor required to get it all in and working properly. Obviously if you don't need quiet (squeaks, rattles, banging), or capable (trackday, drag strip or autocross with no mechanical consequences), or fast (stock small block V8 with maybe 300-350lb-ft), then the expenditure goes down. It all depends on what you want it to do.
I had a series III XJ6, did the mild stuff, some interior work, suspension, etc. I had a stout Dart headed 406ci small block ready to go in it, but I decided not to pull the trigger and sold the car, primarily because of the effort required once the engine was in to get everything else up to snuff. Not that building a proper set of headers for it would have been easy, but in particular, the rear suspension and the lack of off the shelf hardware to beef it up, particularly because in that system all torque both drive and braking, is taken by the I-shaped lower control arm, the same as an E-type. But E-types don't weigh 4000 lbs, and very few make no less than 450 lb-ft from barely above idle. I have the machinery and tools to make the things required to remedy the situation, and other people have done it, but for that amount of effort, it's really a labor of love.
Bottom line, if you don't mind a loud, rattly, poorly air conditioned british saloon hot rod, then yes, an XJ6 makes an excellent platform. But if you want something that ends up like a real car, a normal car except with huge torque, capable suspension and the (arguably) best looking curvy/modern sedan body style of all time, the be prepared to spend resources on an appropriate scale. Here's mine when I sold it: