I heard REF calling!....
We have done exactly what you are contemplating. Our build was done precisely because we wanted to be able to carry our "carbon fiber children" inside the van and have them available wherever we travel. At over 50,000 miles to date this has worked very well for us. We are headed out this morning for a 2 week trip through southern Arizona so this post will be short. I can answer direct questions through a PM and can provide sources for fork mounts and wheel carriers. Some points that I can make now:
1. Our Sprinter is an EB, an LB would give you additional length but might not be necessary.
2. The "wheel well to wheel well width on a 3500 is not enough for 4 bikes, a 2500 works well.
3. We set the platform to give us 40 inch height in the garage. This results in a 26 inch height above the mattress in the bedroom, perfect for us.
4. Checkout my gallery for some photos. More can be found in the early posts of our blog:http://www.travelsofthemercury.blogspot.com
Good luck on your build and welcome to the spending club!