Why the 600 pound load rating? Are you planning to hit some wild jumps in the van?
I'm not 100% sure how it works; Do you drive while in the chair or is it stowed behind the driver seat? If you're sitting in it then the 600 pound rating isn't too far off. If not then 200 should be fine. Why an electromagnet? One of those quick release NdFeB assemblies would be much easier to build/install, and less complicated to operate. Just make sure you can reach the lever!