What year is that SC300 that your friend has for sale?? How many miles? I dont think it is worth 5k, it might be 3.5k. The sound system you can get oem used "fairly" cheaply. If everything else except the engine/tranny is messed up, I wouldnt buy it unless you're gonna have the finances to fix it. And this can be costly (trust me I know). Lexus parts, even though its a Toyota really, are expensive, even used. If you're gonna replace inside trims of your car, its gonna be tough to find them cheap. Check
www.car-part.com that all body shops across the U.S. use to locate used parts for their customer's cars.
Other than that, supply us with some pix and lets see whats gotta be done.
Cheers!