Jet,
I would recommend a plastic canoe over aluminum. The aluminum models just make a lot of noise on the water and are not as "stealthy" as the plastic models.
One thing to look for if you want to use it on a lake, and particularly with any type of motor (more on that below), is some type of keel on the bottom. A whitewater canoe will have a smooth bottom to provide quick maneuverability, but on a lake, it is better for the canoe to have the ability to "track" on a straight line, and the keel will provide you with that. On a plastic canoe, the keel is usually a rounded hump along the centerline of the boat from bow to stern. That being said, if you find a nice one without a keel, you will be OK. A keel just makes things a little easier.
If you can find a used Old Town or Mad River canoe in your price range, go with those. They are extremely well made and will last a lifetime. The hulls are laminated with ~5 layers of a variety of plastics, and the gunwales (the strips along the top edges of the sides of the boat) are usually oak (nicer models) or high impact plastic. Most (not all) of the older Coleman canoes were very cheap and won't stand up to much abuse. The thing to avoid on these is a tubular aluminum internal frame, with one tube running the length of the canoe in the center bottom. If it has this frame, don't buy it.
Using a motor on a canoe is a great help on a lake. There are some nice little outboards that work really well, but an electric trolling motor works great too, as you can set up with the battery behind your seat and out of your way. They are much quieter, too. You do not have to get a canoe with a flat stern to be able to use a motor. They make motor mounts for canoes that mount across the gunwales behind the rear seat and allow you to mount a motor on the right or left side of the canoe.
Here are a couple of examples:
LL Bean Motor Mount
Cabela's Motor Mount
Spring Creek Outfitters "Flipper" motor mount
If you do go with a motor, remember that you need to register your boat with the DMV. If you just use paddles this is not necessary.
I second Beerfly's comment on the paddles. The wood paddles just won't hold up over the years. Get a pair of good quality aluminum/plastic paddles. I have Cabela's 1000 series paddles and could not be happier. The shafts are plastic clad aluminum, so there is no aluminum residue on your hands. The 400 series has the bare aluminum shaft, but they do have a protective casing above the blade to keep your hand clean.
Cabela's Paddles
If you can get over to the Roseville REI, take some time to learn about the different features of the various canoes they carry. They have some good literature that compares the various features.
Oh, don't forget life the vests!
Hope that helps,
Buzz