The best source I've found for interstate rules is the FMCSA website:

http://www.fmcsa.dot.gov/rules-regulations/rules-regulations.htm

I was at a Colorado Patrol presentation, but it has been a few years ago, so it may be out dated.  The trooper advised the locals at the meeting that once they loaded their horse into their stocktrailer and headed for a rodeo, their "ag" exemption was no longer in effect.  If they were headed to work cattle as part of their ag operation the ag exemption applied.

I've not studied what the "Not for hire" designation requirements are.  If you have a USDOT number and run intrastate for years, and then become engaged in interstate commerce, you will likely be considered a "new applicant" and be subject to an initial onsite review/inspection.  As I say, I don't know if the "Not for hire" status will cut you any slack, either regarding the FMCSA inspection or the vehicle markings.

Hopefully the link will be helpful.  When you get the USDOT number, they will probably send you specifics about markings, at least regarding size and placement.  My truck is marked with "VIN  **123456"  (the last eight of the vin)- I've not had a port question it.  Also, I think I would go with the IFTA rather than pay trip permits.  I believe the IFTA is $50 + $2 for the stickers.  Yes, then you will need to log your miles - you may need to do that regardless(??) - and file quarterly reports on state miles travelled and fuel purchases.  But you will not need to look up the permit station every time you enter a different state.