Personally I would get it braked, it just makes it easier to shoot. off the top of my head I am wanting to say that it can be done for less than 200 but that is just a guess. If it was me, get a 300rum in a sendero, have the trigger tuned, have it bedded, and put a good brake and you should be race ready. Me and my dad shoot alot of the senderos and they all shoot good. I have also dealt with the savages and they shoot well to but I am more or of a remington type guy than a savage guy.
Is it necesasary to do the work to your sendero to get it to shoot good. It shouldnt be but you will be alot happier with it. Oh, I would go with a holland style brake but the vias type brakes work well to.
I am building two 300rums right now and will tell you how they shoot later. One for my dad and another for my dads partner because his wife wont let him have his old 300rum in an older sendero back and he has to get a new one. By the way, two years ago she took and elk out with it at close to 600yrds with a neck shot. It wasnt luck either, "Honey, im going to shoot it in the neck." "You sure?" "Yeah." neck shot at 575 or around there. His has a trigger job and a brake.
I dont know who ever came up with the imaganary word of "Over Gunned"