In your case, a 16 would be a good beginning boat. It can hold two people easily (three and it gets crowded and don't expect to go as fast), they are inexpensive and plentiful, they are easy to set up and a blast to sail. They don't have daggerboards to wreck and are made to sail onto a beach. And, if you don't like it then you can sell it pretty quickly for roughly the same price you paid for it. If you find it's not enough boat for you, you can sell it for roughly the same price you paid for it, and use the money to buy a bigger, more advanced model.
I love sailing my 18 more than the 16, but the 18 comes with the price of more hassles in rigging and parts. The 16 is rigged in no time. I'm confident sailing the 16 solo, but am still building confidence on sailing the 18 solo (it requires more effort to right than the 16).
The 17 is designed mostly to be sailed solo, but taking two is not unheard of. Also, does the 17 you're looking at have a jib? That's important for tacking the boat, and without it as a beginner you're likely to be frustrated.
For a beginning sailor the 16 is recommended. Click this link for a good primer on buying a cat:
http://www.sailingproshop.com/catamaran_guide.aspx