One consideration is the screen size. There are some different sizes among the Blackberry devices.
For example, the latest Bold 9900/9930 is 640x480. Some earlier Bolds (9780/9700/9650) are 480x360.
The Torch 9800 is 360x480 (or 480x360 when rotated). The Torch 9810 is 480x640 (or 640x480). These are the slider models, by the way.
The all-touch Torch models (9850/9860) are 480x800 (or 800x480).
The most recent Curves (9350/9360/9370) are 480x360. Some older models (9300/9330/8520/8530) are 320x240.
The above screen sizes are gleaned from the BlackBerry website.
Going from these numbers, it looks like the minimum screen width you'll want to aim for would be 320px - if you care about accommodating the older Curves. If not, move up to 360px.
Aside from screen size, you also need to consider the browser support for web standards. Prior to OS6, the BlackBerry web browser is awful. It's noticeably better in OS6 and OS7 (based on Webkit). Take a look at the Mobile HTML5 site for compatibility tables.
You don't necessarily need to buy multiple devices for testing - there are Blackberry simulators available to download and run to test your websites on. I haven't used these, but they are supposed to be useful for testing!
My suggestion would be to ensure you are using mobile-friendly layouts, optimize for the latest browsers on OS7, and ensure that the site remains usable on older devices. the Bold is very prevalent, so pick one of those, and one of the Torch models, and possibly a Curve.