For $1500 you might be able to find an old 14' riveted aluminum with a trailer and motor on their last legs. BUT, it could definitely be worth the money.
Off the top of my head:
1. You want at the least a 15HP motor. A 25 will scoot along just fine with 3 people in the boat, but a 15 will be slow upriver.
2. When you test drive it, be sure to put forward and backward pressure on all the seats if they are riveted through the hull.
3. When you get to the boat launch immediately get out and check if the trailer bearings got hot.
4. Ask how much the boat has been in saltwater. Salt is bad, for both the motor and the trailer.
5. Check the prop and ask how old it is. A beat up prop can ruin a lower unit.
6. The boat will probably have plywood on the transom. Use a screwdriver or something to make sure it isn't rotten.
7. A wide beam/bottom means more stability. And the more freeboard the better. (freeboard= water level to gunwhale)
7.5 Trailer lights and wiring.
8. If you buy it, have the trailer bearings greased and probably replaced. Have someone service the motor and probably replace the impeller.
9. Replace the gas tank if it's an old rusty one.
10. "Extras" can add up to a chunk of change if the boat doesn't already have them....Life preservers, rotating seats with backs, anchor and rode, fire extingguisher, rod holders, air conditioning and power windows....
11.If you can, take someone that knows about boats with you.
I'm sure I'll think of some more during tomorrow's hangover.