If you're able to get a Revision 0 that's in good shape, go for it. I waited until I found one before I jumped on the Dreamcast.
Revision 0 has a pretty reliable disc drive, and more importantly has a metal fan and bigger/better heatsinks.
You can tell which revision the model is by looking on the bottom sticker: there's a number in a circle in the bottom right. If you can't find a Rev 0, grab a Rev 1 that says Made in Japan, it's the next best model. DO NOT grab a Rev 2, as they're made with cheaper parts, and cannot read backups.