Shall I assume from your response you would not buy an after market barrel but would rather be equally satisfied using a factory barrel? Also, the 550 in 375 comes in a 25" tube. Would that be too long for all round usage?

Yes, Keep the factory barrel - after all, why change when they work. And, if you buy the gun as the basis for a cusom project, you cn shoot it first to see that it shoots !

25" would be a great barrel length - nice and dandy IMHO.