Generaly the barrel will come to you just as a contured blank with the bore rifled and lapped , no crown , no chamber and no threads. You can order some that way for extra cost but I think your money is berrt spent with the smith actualy installing the barrel to the action.
I would think that the only thing between the 308 and the 30-06 case would be the 7x57. The 7x57 case is the parrent for the 6mm Remington , 257 roberts and several other wildcats , this case does have a bit more capacity than the 308 cause its longer but thats about it.
Improving a case to a 45 deg shoulder would be a bit steep for my liking , like you said the chance of colapsing the shoulder is alot higher.
I'm not sure exactly what it is that your wanting
1) if you want a 25-308 improved great have one built , but know that
your not going to run a 120gr bullet out at 3200fps , maybe once or
twice but not for long before somthing breaks.
2) if your wanting to run a 120gr bullet out at 3200fps then you need to
look at the 284 case as a minimum and a long barrel , the RSUM or
WSM would be a better parent.
But to answer your first question about the barrel twist , a 1-9 will work fine and many barrel makers offer that twist