Um, I'm pretty sure that the IS300 has a 2JZ-GE in it, the only thing I can really tell you is when looking at it make sure the oil cap is black or golden, not milky. Try to get a compression check done and make sure that radiator fluid is green. Also, once the car is warmed all the way up make sure it idles solidly at 700 w/o AC and 900 w/ AC on. If it fluctuates a lot it could mean a sensor is out of alignment somewhere. The 2JZ is know for bad head gaskets, not as bad as it's little bro the 7M, but still many are known for losing head gaskets and valve gaskets (which regrettably are really hard to change thanks to the infinite wisdom from Toyota, the coolant is routed through the TB which has to be taken of for such things as valve gaskets or spark plugs). Also I don't think the Navigation would add enough weight to ruin gas mileage.