Comforting words here =>
https://ralphhosier.wordpress.com/2011/10/02/de-mystifying-the-jaguar-v12/
If you haven't already got the engine and the plan is to use as much of the original systems as possible I'd be looking at the most recent versions
If you are used to American V8 spares prices I do think the costs of buying Jaguar bits will probably make your eyes water. I can't imagine for a minute it will be cheap especially if machining work is necessary.
There's a chap over on Landyzone who is playing about with a V12 in a series Land Rover - I can find the link if you want