Your current carrier or the carrier you will run with is the first place to check. Carriers offer varying levels of help with permits, base plates, etc. Check with your carrier first to avoid obtaining permits or doing paperwork yourself that your carrier may already provide.
My home state (MN) publishes two manuals that contain info about the quesions you raise. Perhaps your state has similar publications
International Registration Plan (IRP)
International Fuel Tax Agreement
The web site where they post them is defective. The link below will take you to the wrong page but it's close. Click on the link below and then click on the "Prorate Registration" link to access the above mentioned manuals.
Yes, there are services that will help you with compliance and paperwork. One is called DOTAuthority.com. I know nothing of them beyond what they say about themselves on their web site.
http://www.dotauthority.com
Has anyone out there used such a service? How did it go? What would you recommend? My wife and I will be going through this process in a few months.