Adam,
I think your question is how can you confirm the car matches the VIN on the paperwork and if it is important to have a VIN tag on the car -- based on this understanding, here are my thoughts.
My opinion is that a missing VIN plate is not a big deal if you confirm the car matches the paperwork. Sounds like you've done some work to match the VIN. The VIN is stamped on the right side of the car in the engine bay under the throttle body --- look for the flex air hose from the air filter to the throttle body and look underneath --- bring something to spray it clean and a flashlight. See links below for location. This VIN obviously should match the VIN on the paperwork and on the paint data plate you have already found.
In addition, since this is a 250sl, you can also check if it is an
early or
late 250sl as they are different --- and it's just another point of validation --- it's a very loose validation but if the last 4 digits of VIN are 2979 or lower it's an Early 250sl and if these digits are higher, it's a Later car. If the VIN is 2979 or lower, look for characteristics that match a 230sl, like a squarish dash, 230sl inside door handles, chrome inside mirror, curvy outside mirror etc.
If the last digits of VIN are 2980 or greater, look for details that match the 280sl --- rounded dash, rectangular inside door handle, non-curvy outside mirror, black inside mirror etc.
Look at photos of 280's vs 230's and you will see what I'm writing about -- look at door panels, dash, mirrors and it will make sense. Of course a few of these items could have been changed, but changing a dash is no easy feat.
See the photos in this link so you will know how to find the VIN on the frame. Let us know how it goes. Good luck.
http://www.sl113.org/forums/index.php?topic=6345.0http://www.sl113.org/wiki/DataCard/VIN (go to the bottom of this page in the tech manual)