I believe what people are calling a hard drive is a CD-based system while the later systems are DVD based. A single DVD will hold much more data than a CD so that you can probably get the nav data for both Canada and the US on one DVD where it might take 9 CD's just to do the US.
If a Navigation system is important to you, get one with a DVD-based system.
I'd look for a one-owner car with a solid maintenance record. Paying a little more up front could save you a bundle later on. Also insist on or buy a CarFax on the vehicle because you'd be amazed what you can discover with a CarFax. If you buy the car from a dealer, make him pay for the CarFax and show you the maintenance records for the car.
A CarFax will show you how many previoius owners, any accidents, whether the car was privately owned, leased, owned by a business or a fleet car, if the car has been repossed, where the car came from, etc. Obviously, you want no part of fleet cars or any vehicle where many non-owners have driven it.
Don't even think about E-Bay unless the car is local to you so you can check it out.