Well to start with, there is good and bad to both.
The Savage right out the box is not going to be anything near what a custom action is. To start the Savage action has a horrible bolt lift, it can be fixed with a Time and True (T&T) job from Sharp Shooters Supply, (SSS) and you will have a bolt lift as light and smooth as a custom action. Now you have to figure that cost ($125.00) and time into this. The second problem is the savage doesn't have a trigger that is even close to comparison to a nice 2 oz Jewel. The target accu-trigger is a complete wash. Now you can spend money and get a SSS evolution trigger which is close in feel to a Jewel 2 OZ trigger again now you have to add that cost into this. ($195.00 on top of the mandatory T&T job to get this trigger.) Now that you are at this point, you are close in cost to a new Kelby Atlas, or Stiller predator, without trigger. The Savage will hold its initial value, but you can not add the work to the price, it will be worth about 75% of what you paid cost of the action (Without work or trigger.) barrel and stock. The biggest problem with trying to make a savage fit a IBS/NBRSA 100-300 yard light gun class is weigh, and lack or light weight stock. Most stocks are laminate, and are just not suited for this type of shooting, but the do do well in long range IBS/NBRSA shooting.(600-1000 yards.) Now a skilled gunsmith can get a un-inletted stock that has a Remington tang to fit, and you would never notice, but stocks are not cheap, and inletting adds to the cost. You are also limited as far as going with a PPC, needed a special bolt head, only available at SSS. Just remember when you get done it is just a Savage.
Now moving onto the custom actions, you have to remember just about any used rifle you buy will need a new barrel. You are possibly getting someones problem child as well. You could get a tempermental gun that will make you want to bang your head against the wall, it may have to go to a gunsmith. The gun will be worth what you paid for it. When you figure in the cost of a used gun, and getting a new barrel for it, your almost at the cost of buying a new gun.
My suggestion would be to go to match's and buy a used gun that you can shoot before buying, or you know that it is good. Or take a bit more time and save a few more dollars and buy a new gun. Just going with my experince, the savage works great as a long range gun, and I would highly recamend building such a thing. Now I build a IBS/NBRSA 100-300 yard gun that will fit into a 10.5 light gun class, and I love this gun but I would not do it again. Dont get me wrong the gun is super competitive, but I prefer my Bats for this type of shooting.