Hi,
From what I hear the XJC is the mostly likely Jag to experience substantial price rises in the future.
As always it is the best in original condition that will command the stratospheric prices while the modified will be left behind in the wake.
I am quite new to Series II Jags having just bought, on impulse, a 78 Daimler V12. I am very familiar with Series IIIs, XK8s and X300 Jags. First impressions? The Series III is a vast improvement on the Series II in many ways. The mechanical excellence, poise, balance and beauty of the Series II is largely let down by poor build quality and poor fittings hidden beneath a good looking interior. The XJC is no doubt a great car but for me, my choice would be a late Series III Sovereign with either the 6 or 12.
Our X300 lacks the real beauty and 'personality' of the Series cars but has been the most wonderful robust and comfortable cat. What a shame the series cars did not have the quality of the X300

not sure if this helps but above all, if you go ahead, stretch yourself and buy the very best one you can afford. Quality is remembered long after price is forgotten.

regards
Al

Hi,
I am far from an expert, in fact I'm really just a novice but I won't let fear or lack of intelligence prevent me from offering you an opinion
The 4.2 XJ 6 engine is a robust lump of iron and aluminium that is not only relatively straight forward but is backed up by a huge supply of reasonably priced and a wealth of knowledge.
It therefore might be relatively inexpensive getting the existing engine back into reasonable condition with added bonus of retaining your car's authenticity.
As you state that the engine is running albeit smokey perhaps even a 'short back and sides' (rings, bearings and a head job) may vastly transform the car for a minimum of expense though others on this forum might decry it as a cheap fix.
As for the gearbox, again they are common and available, so whether you have yours rebuilt (my choice), get a reconditioned exchange or take a punt on a second hand one, you have practical and affordable options available.
Of course, to go down the above path is to bring the Jaguar back someways to it's former glory.
To pursue your other ideas, that is to install a V12, V8, triple expansion steam engine or nuclear reactor is to go down a completely different but equally valid path. If that was your decision, how far towards a hot rod you go is your business and dependent on your time, your wallet, your commitment and how drivable you want the final product to be.
If the job is poorly done your car will basically be a wreck, if well done, a fine machine but of little relationship to the original car.
My feelings is that to re-engine purely to replace a smokey existing engine would be a very complex and frustrating way of reinventing the wheel. So I guess in a long winded fashion I believe you are making the correct choice. As to what head you use, very much your choice. Many fine cars have used carburettors so injection is not essential but I'm sure with a little effort you will have a car which is elegant, fast ( in context) and a pleasure to drive.
Al
regards
al