Thick brown sludge at thermostat housing
 

Hey everyone, new to the forum and I appreciate any advice. I have a 2004 montero sport that I’m trying to get ready for my son. It has driven before, but had various issues with idling and then developing a hole in the radiator. Not sure who designed the plastic radiator, but replaced the radiator, then developed a new hole by the cap a few months later which caused me to just park it for a couple years. Just patched the hole with some high heat epoxy putty and started to add a little coolant and went to replace the thermostat, and a thick pile of dark brown sludge rolled out of the thermostat housing (SEE VIDEO). Obviously, I’m concerned for a head gasket issue or cracked head, but no milky residue on the oil cap, although the oil is a little bit low. No milk in the transmission fluid. Any chance this is from possibly mixing coolant types, it overheating after the leak by the cap and then sitting for two years? or are my initial fears justified and this is for sure a head issue? Thanks everyone for your input!