How many are we talking about and what kind of approximate age will these computers be?
If they arrive without OS they can still be booted. In Target Disk Mode from another Mac or from a Mac Os installer or bootable system on an external drive for example. Depending on the age the most common way to start up among them may be USB. If you have or create an USB Installer from Mountain Lion or create a Mountain Lion System on (a partition of) an external USB disk you can use that to start up a whole range of the most current Apple computers.
The ones that don't start that way may require older system versions, won't boot from USB but may have Firewire ports or optical drives to start from installation CDs/DVDs.
Starting them up that way you'll be able to find out things about those computers that aren't visible on the outside. When started up that way you only have to go to About This Mac > System Information or System Profiler (Utilities folder) to get to see if the model had Additional Ram installed later, user upgraded HD (size/spindle speed), User installed SSD etc when comparing with the Everymac data posted above.
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Ask the Refurbishing company to attach a data sheet for each Mac they sent you.
What will you be doing with these Macs and what kind of arrangement makes a refurbishing company ship blank Macs to you?