Thoughts and recommendations on e320 engine swap (W124 wagon)

[92-2.6]

The head gasket in my '94 E320 wagon gave up recently. The original engine has 260K miles, and while it hadn't overheated it did get very close on two occasions, and the oil pressure had dropped very low. Also, the transmission has become slow to engage reverse on occasion.

I picked up a supposedly 112K mile M104, and again supposedly 26K mile transmission last weekend. Both were already pulled before I acquired them, so there was not possibility to check their condition prior to purchase. Fortunately the price on each was reasonable, and I'm hoping for the best.

My question for the community is given the M104's traditional head gasket woes, and with an unknown history on this engine would you replace the gasket prior to installation into the car? Anything else that would be best to address now? Also, while the transmission looks great, and the fluid appears to be good what would you do while it's out of the car? I'm certainly planning on replacing the rear main on the engine and the input seal on the transmission, but anything else that I really should consider?

Both the engine and transmission were sold to me as being from a '96 model year w210 E320, but everything I researched seemed to indicate that these should go into my '94 W124 chassis fine.