I have to second Doc's opinion, if you plan on heading out more than a few miles, a nice heavy 19-21 would be the thing... I do wish I had a 19-21, but to get a reliable motor with a 19-21 for what I paid ($2500) is near impossible, so I went aluminum... plus as I said, I originally planned on selling my Camping trailer, buying a slideon camper, and towing the boat to Montauk (camper/boat combo) and launching from the beach... (that's the other reason for a lite aluminum).
If you can afford it, go for the bigger boat, you probably will want it in a year or two anyway after you get tossed around in the aluminum a bit... but then if your planning on buying a house... etc... and aluminum is all you can afford or want to commit to right now, that's understandable too. I wanted to point out the positives on the lower cost solution...
If you go for a fiberglass... and find one for a lower price... You main worry is the transom, after you find a reliable motor. My neighbor put a $12,000 ouboard on an older boat and blew out the transom. He now has an outboard in his garage, no boat.
...and just to put things into perspective for you, an equivalant Yamaha 40 to mine, new, is around $4500-$5000.
As far as checking the rivets, just see if there's a lot of sealer on the bottom of the boat, or a lot of them replaced with screws. Check for galvanic corrosion (*********** or crust) dents, dings... and check the wood on the transon of the aluminum too.
I always check the weather before I head out, and only use it if forcast calls for 1-2 foot maximum (there's always 3 footers out there too in that case) Today, because of the wind, I would never consider heading out. The day I took the trip to Middle grounds, the trip back to Kings Park (10 miles) took 25 minutes at 25mph. The day before I went from Crane Neck to KP (about 6-7 miles) and it was rough, lots of 3 footers, took about 45 minutes and I looked like a giant salt lick by the time I got to the dock.
Just trying to tell you what to expect.