To check for codes, locate the X11 port. I think it's somewhere along the firewall in your TE. There's a port with a rectangular flip top cover. With the key to on, engine off, press the button for 2 seconds then count the flashes of the LED. Press the button for another 2 seconds and count the flashes. The codes will display in flashes in ascending order then repeat. To erase the codes, display the code then hold down the button for 10(?) seconds. You have to do this for each code to be deleted.
One of the built-in safety features of computerized engines is that they retard timing when a fault is detected. That might be why there hasn't been pinging since the check engine light came on. Could be a thousand other reasons. If you've had an oil consumption problem for a while, the deposits in the combustion chamber could be contributing to pinging.
Pulling the engine and transmission is a big job, much bigger than dropping the transmission and pulling the head. The lower half of these engines is near bulletproof. I wouldn't touch it just because it has 150K miles.
You can leave the manifolds in the car when you pull the head. It's usually cheaper to have a valve job done to your otherwise good head than to get a rebuilt head, without factoring in convenience.
Consider a new timing chain, tensioner and guides while the head is off. The in-line engines are much easier on the timing chain with fewer direction changes than in V8 engines but it's an opportunity. So with the water pump.
I can't advise you with the transmission. I don't know how tight a W124 transmission hump is. There's access galore in a W126.