I would install a new tb and all the crossover parts involved. Plugs, Clean the fuel system etc...Change all fluids, Brakes, Radiator etc...Lot to be done but worth it on a 13000 mile SC400
Mobil 1 5w30 Synthetic every 5k regardless of driving conditions, I always used to drop the couple quarts of tranny fluid at the same time I do oil, Keeps the tranny fresh and reliable and very cost friendly...