Yes, imaging is a good idea, even for a small number of machines. You are getting some amount of time savings by not repeating tasks, but you're also getting consistency and the ability to re-image a machine if it needs an OS reinstall. E.g. virus/malware, drive dies, some major file corruption, etc. Our policy is if it's going to take more than 30 minutes to troubleshoot and tweak to fix an issue, it gets reimaged instead.
Regarding the licensing of imaging with OEM licenses, see this Msft article for some more details regarding this scenario:
Re-imaging rights are a benefit granted to Microsoft Volume Licensing customers. Microsoft Volume Licensing customers may use Volume Licensing media to re-image software (including OEM Software licenses) under the following conditions: The copies re-imaged from the Volume Licensing media are identical to the originally licensed product (the same product and version, contain the same components, and are in the same language). The customer must purchase at least one unit of the product required to be re-imaged through their Volume Licensing agreement in order to obtain access to the product media and receive a key. Volume Licensing media must be used for re-imaging (OEM media may not be used).
In short, if you own at least 1 volume licensed copy of Windows 7 Professional, you get access to the VL installation media and VL product key for Win 7 Pro. You would then use that to do the OS install and imaging. The single volume license is essentially just used to get access to the VL media and product key. This applies when your OEM-licensed product is available as an identical volume-licensed product. E.g. Windows 7 Professional.
You can't use the above scenario for OEM/FPP Msft Office licenses because volume-licensed Office doesn't have the same versions as OEM/FPP. For example, there is no Professional or Home & Business version available through VL, only Pro Plus and Standard.
Regarding your question #2, your company purchased the machines, the legit COA is on the box, you're good. OEM licenses live and die with the machine itself; person who owns the machine owns the OEM license.
Regarding your question #4, install every possible thing you can before imaging! The exact software that can be installed depends on your specifics apps, but for all of our LOB apps and such we install all of it before creating the image, as none of them require unique serial numbers or anything like that.
For unique/specific licensing questions, I always call Msft licensing people directly: (800) 426-9400
Once they provide you with an answer, you can ask them to direct you to some sort of written reference that confirms what they tell you.