[Tristan Lall]

Do you have any special requirements for purchasing? For example, does your school district have preferred suppliers, or require a warranty? Also, how eager would your students be to build (and support) computers for the team?

For CAD, you should be looking for a processor with the fastest single-threaded performance you can afford (so probably an Intel Core i5 or low-end i7). For 3-D rendering, speed is still useful, but you can also take advantage of multiple cores. A mid-range consumer video card will generally be fine, unless you're aiming for high-end work—in which case the optimized drivers of a low-end professional-grade card (i.e. with the same specs as a mid-range consumer card) will help significantly.