What Larry said is true. Rotomolded bodies are fairly similar in their construction. But make sure the chassis is solid and well-planned for usability and ease of access. Sandia does a good job designing a smart layout.
Now, what's under the hood? At the end of the day, that's what really counts. At the core, it comes down to vac motors, pumps, and fittings. What's been chosen with the machine's design for its intended application? Is the component selection high quality? Are the internal parts laid out intelligently? Again, a nod goes to Sandia's extractor design.
And finally, how will the company support the end-user if there's a problem? We've worked with Sandia equipment for the past 15 years or so, and I have to say they've ALWAYS gone the extra mile to take care of our customers. From our experience, I'd say they've got a solid reputation.
Low price? That's Sandia's icing on the cake - manufacturing a solid unit and bringing it to market at a good price. The bottom line; their approach to the portable market has been well-executed.