+1 the trans stat will almost certainly fall apart when you pull it out, look very carefully for the locating tab when you install the new one. Nissens is really good so no problem with that for the radiator. The new coolant sensor usually comes with the expansion tank.

As far as the cooling system flush, back in the day it was usually some combination of corrosion, and silicate in the antifreeze to protect against the corrosion that caused the buildup that needed to be flushed out. These days when the (bmw recommended) antifreeze is silicate free and still protects everything and everything is aluminum I think way less of a problem, unless maybe someone used the wrong antifreeze or added something that shouldn't be in there.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)