1989 Jeep Comanche idol and overflow tank

I just bought this cute little jeep. When I test drove it 2 days ago, was fine. Today, it sounds so loud and when put into drive, don't even have to press the gas - it goes. Is that the idol?

Also - the radiator overflow tank. Why does online say $25, local dealers between $70 - $120. Is that because the place is of better quality or pressurized?

Opinion please before I take it to mechanic and get ripped off?

Thanks,
Nancy