Last I knew, a class 8 tractor with trailer required a class A, class B is commercial straight truck. If you register it as a farm truck, you typically are limited to a relatively short range around the farm, like
150 mile radius. Insurance is another headache, though since you have commercial insurance already, probably less of a pain. You need to check with your state motor vehicle department to find out the particulars for your state.
Even with the reduced rate for farm plates in my state, plates still could not be considered cheap. In this state, the farm plates allow certain exemptions from the cost of the Commercial carrier's record keeping requirements, however, The feds have recently determined that this is a violation of the Federal Motor Carrier rules, so, even with the exemptions written in the federal rules they will pull the state's federal funds unless all the farm exempt privileges go away by Dec. 31, 2009. Check your own state rules, and, local state farm organizations. A road tractor which is a good deal today, might be a time bomb. In this region, the determination, is going to require, all farm equipment over 17,000lbs, to need a CDL operator to move on the highway under its own power. Check your local rules.
I am not sure if the exemptions going away, is going to be a nationwide thing, or, done on a state by state basis. I spent a good share of last week sorting out the rules, as they apply to me, within my state. My trucks will be parked.
Just keep the federal tax in state and repair roads with it. The problem is the Fed's take the money and take 20% or so as management cost and send the money back with lots of strings attached. Like make the highway speed 55 or none of that highway taxes paid in your state will return to your state.
Minnesota does still have the farm exemption, at this time. However, as you noted you still have to comply with DOT and all of that fun. As long as you are transporting your own product you are exempt from the majority of the regulations.
You do still need a health card to pass CDL requirements, but don't need an actual CDL.
Your equipment does still need to meet all DOT inspections, which is an annual thing. Couple hundred $, for the tractor and trailer.
You will have to file for a DOT number. I don't think that cost much. You only need one, and it covers your entire operation.
Licensing will vary depending on the gross weight you license for. Done on a quarterly basis. 76k gross is about $400/quarter. 55k is about $200. (I drop mine down in the off months).
There are some headaches in having your own rig, but the convenience factor is huge. I have a grain trailer and a drop deck. The drop deck is nice in that if I see a new toy, I don't have to worry about moving it. I have gotten a few "steals", because nobody else wanted to mess with a machine because it was too big for them to deal with.
I don't know your market model, but my rig has more than paid for itself in the better market price I can get past my local elevator. I can truck directly to the river port and gain quite a bit in market price.