Paul,
I grew up boating on the Columbia & Willamette in boats just like the ones you are looking at. I have a couple of thoughts if you are interested...
I would stay with a fiberglass hull. (welded aluminum is waaaaaay out of your price range, I don't trust riveted aluminum and wood is just too much maintenance.) Don't get anything smaller than 16' and, really 18' is just about perfect. Boats get a LOT smaller when they are off the trailer and in the water! A 'V' hull will ride a little softer, but a trihull will be more stable.
The Willamette is narrow but deep and on the weekends gets pretty crowded and tends to be choppy.
The Columbia is much wider, but (outside of the shipping lanes) can vary from 4' to more than 90' deep and in the early evening (5:00ish) tends to get a North/South wind that blows up and can also get choppy. The Columbia is one of the most treacherous rivers in the world. The current changes from 2knots to 5knots throughout the course of the year. The sandy bottom is CONTINUALLY changing and new sandbars pop up all the time. It is also a working commercial waterway, so you are going to have to contend with tanker ships, cargo ships and a constant flow of barges.
Outboard motors are simple 2 stroke, water cooled, 2, 3 or 4cyl engines. Nothing mysterious there. But you are correct in your assessment of inboards- they are the same as their automotive counterparts (except for the cooling system which is also pretty simple), so propulsion is really a matter of taste. Outboards will get you up on a plane faster, but don't idle very well. Inboards get better gas mileage but take up more space. If you do go with an outboard stay with OMC (Johnson or Evinrude) or Mercury. Everything else is crap. (oh boy, here it comes....) If you want to fish (trolling rig) you will also need to get a smaller trolling or kicker motor (3-15hp depending on the displacement of the boat)
Make sure whatever you get has a title. All boats over 10' and/or with a bigger motor than 10hp have to be registered. For that you will need a title. Getting a lost title for a boat is the same as getting one for a motorcycle. PITA. You will also need to get your boaters license. As previously stated, you can get it online. (I would also agree to either take some classes, or go out a couple of times with someone who knows what they are doing first. Remember, 6 or 7 people die every year in those two rivers.) You will also need some 'stuff' to make the boat Coast Guard legal: Life jackets for everyone on board; a horn of some kind (handheld air horn is ok) and a throwable (something that floats with a rope tied to it).
Also remember to check out the trailer: lights, tires, overall condition, etc... And practice maneuvering it before you take the boat out your first time. Launching and loading are trickier than you might think. Do yourself a favor, spend a couple of hours @ Chinook Landing or the 42nd st ramp on a nice Saturday and just watch the chaos.
If I was looking I would be very interested in this:
http://portland.craigslist.org/clk/boa/1870722526.htmlPlus he's been trying to sell it for a couple of weeks and may be flexible on the price. I have no association to it or the seller.
Sorry this was so long winded.
After all the help you've given us over the years, I would be remiss if I didn't offer to reciprocate: I'd be happy to take you out on our boat (17' trihull w/ a 75hp outboard) so you can get an idea of what's what. Or if you have a boat in mind, I'd be happy to come with you and take a look and give you my impressions or give you a couple of suggestions of what to look for.
-JP