If you were trying this with a new motor and trans, well, set aside about $70,000! Here are the numbers I am coming up with. I am just using current prices from Ebay.
5.5L V12 w/turbos and intercoolers 60k miles range- $4800
Porsche tiptronic- $3500
Ecu + harnesses- $12,000
Slush fund of no see'ums- $4,000
So, $23,000 roughly and you are driving. You could probably find a touch better deals. You will possibly need to modify axle length, but that is pretty easy.
The trans is already set up for shifting inputs via the steering wheel. Both my Cayenne and SL have them. So, get some actual paddles from another car (or make your own) to send the signal to the trans. Pretty simple really. I do not believe the shifter has a solid connection to the trans with a cable, so you could either put in a Benz shifter, or turn it all into push button on the dash for park, reverse and neutral.
It would make for a very easy to drive car that will literally bounce your eyeballs off the back of your skull. Lol.
The current 1UZ I have still needs to be built up. That will be $6,000 minimum. I need to upgrade trans to a turbo 6 spd box. $8,000. Beefier clutch $3,000. New shifter, $1200. Still need an ecu/harness, $4,000. So, I am at roughly $22,000 that I still need into my drivetrain. I can sell off my current stuff for about $7500.
So, I am money ahead by roughly $7,000 to go the V12 tt route with the tiptronic. That is how it makes sense for me.