I think buying exclusively to trade is unwise---the rules of the exchange game are constantly changing, and what used to work last year may not work next year in obtaining the weeks you want. Even in the several years I've been in timeshare, I've seen several fairly significant changes that made tried-and-true methods for obtaining favorable exchanges completely obsolete, nearly overnight.
If you need an example, ask all those folks who bought II weeks with the expectation of trading into DVC, but there are many others. As another: when Wyndham removed the ability to reserve a specific week for deposit, it
instantly neutered Wyndham's pionts product as a high-quality RCI exchanger.
If you really want to stay at a particular resort semi-frequently, you probably should consider owning there.
If you want to extend your timeshare portfolio, I think it is most prudent to buy into a good (but not necessarily name-brand) resort or mini-system that you personally would want to visit, at a time of year that is attractive to both you and others, and at a price that makes sense when compared to the available rental market. If you do that, chances are good that you will be able to extract value either through exchange or through rental if, for some reason, you'd prefer not to use it yourself. But, if worse comes to worst, you still have a resort that you can use. That's important.
Most importantly,
take your time. It's very easy to buy a timeshare week. It's surprisingly hard to sell one. There's no hurry---join TUG (
www.tug2.net) and TS4M (
www.timeshareforums.com) and read read read, and then when you think you know enough to buy, read for another few months.