There is no such thing as a Street Fighter Alpha 2 cab, they are all conversions from something else. Post a picture of the cab and we can help you decide if it is worth $300.
If it is just a dynamo cabinet like SF2 came in then it is worth about $200 in good condition. Add in the A2 board and you are getting a normal price. I would personally try to get the cab without the board though, unless you care about playing A2.
Moving a cab is pretty easy as long as you have a hand truck (dolly) and a thick blanket to lay down in the back of the van. I don’t know if the van is deep enough though, it might stick out the back. Definitely measure before you try anthing. Laying the cab on it’s back is ok, but do so gently, an impact can affect the monitor. You might even want to remove the monitor before transport if you want to be extra careful.
Adding naomi support to a JAMMA cab is easy, but expensive as everybody else said. You might consider instead of buying a NAOMI board putting a J-pac and dreamcast adapter inside and play the dreamcast version. Also you could play any other DC fighter!
You can buy the stuff for that from www.ultimarc.com
If you are really really brave you could hack a Dreamcast VGA box to output in 15khz and send a signal your arcade monitor could recognize. add in padhacking a couple of DC controllers and connect them to your cabs controls and voila, you just converted your SFA2 cab to MVC2 for probably less than $30.