It sounds like you may have a blown cylinder head gasket. The head gasket goes between the engine block and the cylinder heads, and seals the cylinders up. A leak here can cause oil to seep into the coolant, or more likely, coolant to seep into the cylinders and engine oil. Coolant seeping in will cause spark plugs to fail, general rough idle, steam to come out of the tail pipe, and sometimes smoke. Here is an article with more information on blown head gaskets, including how to test for it:
https://www.2carpros.com/articles/head-gasket-blown-test
I am also attaching an image that is Prodemand's repair cost estimate. This is roughly what you will pay for labor at the dealership for the job. Parts will be an additional few hundred dollars, give or take.
Good luck and please let us know what happens next. We are interested to find out.
Image (Click to make bigger)
Monday, December 3rd, 2018 AT 4:35 AM