Re: Your ideal 2700$ 300 win mag build.... Go
Don't skimp on a "real" custom action. They are precision machined with attention to detail and will have a standardized tenon thread and decent heat treatment. In the future, if the day comes that you or a relative (who inherits it) needs to sell it, it will bring more than any accurized Remington action. Pro gunsmiths admit that they like accurizing Remington actions, since they get the greater part of the pie in the complete weapon, even though the all up price is essentially the same than if one had started with a custom action to begin with.
The only downside is potentially needing to wait for the action, but most of the time the lead time for the action is not longer than for a barrel. With a custom action, the tennon can be spec'ed whereas for a Rem action it will be different or larger than spec on every gun.