Can't comment on the manifold assembly, as my diesel is mercifully lacking this added bit of complexity.
The expansion valve looks a bit daunting on the 124, but is actually pretty easy. On the diesel there is a heater hose exiting the firewall a bit above and in front of the expansion valve. Detaching this an moving it aside greatly eases access. You may also have some vacuum lines which are in the way.
The hoses are held to the expansion valve via a single 8mm nut. Use a plain old wrench to break it loose and back it off. Because you *really* don't want to be dropping that nut, I recommend loosening it the final few turns with the fingers of one hand while holding a magnetic pickup tool on it. I have a pen shaped magnet which is great for such tasks.
Move the hoses out of the way. This may required loosening some clamps on the forward firewall, under the alarm horn.
The expansion valve is held to the evaporator via two 3mm allen head screws. Use a small allen key to break them loose and back them out. The magnet tool is again handy for removing them.
At the very least give the new o-rings a generous coating of refrigerant oil. I prefer nylog, a specialty o-ring lubricant/sealant for a/c systems. It can be purchased from http://www.acsource.com.