-- Last Updated: Oct-22-12 2:15 PM EST --
One nice thing about used boats is that you can usually get a 100-percent return on your purchase price if or when you decide to re-sell the thing. If it gets you on the water and you already are planning that you'll upgrade IF you get bitten by the paddling bug, it sounds like a reasonable choice to me.
I bet your "professional" friend has already given you a list of reasons not to get aluminum. As I see your situation, the reason at the top of the list might be the way aluminum "sticks" to rocks. However, if you take the learning of paddling skills somewhat seriously, it won't be long until you are hitting far fewer rocks than rental boaters and very casual paddlers. Paddle a year or two with improvement in mind and you'll be avoiding even more of them, but you'll still hit a rock now and then, and when you do your boat will often stop dead in the water. When you "upgrade" to another hull material, you'll appreciate sliding over the rocks instead.
If you find two boats that are a pretty equal match in design and cost, with one being polyethylene and the other aluminum, the odds are really good that the poly boat is badly warped, because finding a used one that isn't warped is pretty rare. A warped poly boat may paddle "well enough" in the short term, just as is true of aluminum. Find the same match between aluminum and Royalex, and I'd tend to go with Royalex. But if aluminum is what gets you started most conveniently, go for it.