cummins M11 oil cooler question

Hey all,
Have a 1999 Int 8100 with Cummins M11 celect plus in it.
This would be the first time I do this so would like any advice. I noticed the smell of coolant on the way home Friday. Got home and saw it leaking out the bottom of the hose that attaches to the thermostat housing neck. So next day went out and drained the coolant. Was only gonna drain enough to get that hose off but decided I was gonna put all new in it anyway so drained it all. Glad I did because at the end of it draining I saw a thin layer of oil in the coolant. So I am convinced it was the oil cooler went bad. Also had a friend that is a diesel mechanic that came over and said 99% sure its the oil cooler. He doesnt want to do the job at my place. His shop where he works is suppose to quote it but its been 2 full days and nothing and I need this back on the road. So I can get the cooler and all the gaskets and O rings for about $550 from a place in TX. I looked it over in the cummins manual and doesn't look to bad. So just wanted to make sure if I do this what all I should be doing to do it correctly. I know when I got the dealer quote on parts it was $1006. They wand $837 for the cooler and the rest was various O rings and gaskets and then new studs and nuts for the turbo. Someone told me I should also remove the thermostat housing and then clean and replace those O rings also. Any advice would be appreciated.

 

And does anyone know the coolant capacity