In looking at the ads in the usual places, I found a plane that is very interesting. The seller advised me the useful load was 445 lbs. The plane didn't seem to be overloaded with stuff, so I'm wondering what is a typical useful load in real life. I would like to have someone in the right seat with reasonable fuel.
Welcome to the Club, Tommy. Your post is in the right place, no worries.
The 150's max takeoff weight is 1600 lbs. If the seller said the useful load was 445 lbs, that means its empty weight (with oil and unusable fuel) is 1155 lbs. A bit heavier than most, but not terrible. Take a look at
THIS THREAD where forum members reported the REAL weight of their planes.
Subtract your weight and that of the passenger/CFI from 445, and the result is what you can legally carry in fuel & baggage. If it's too little, lose some weight, carry less fuel, or ditch the passenger

.