I'd go custom by the time you buy a 700 for a donor you really aren't going to save much. If your budget allows, look at Defiance. I have a 300 WBY with a Stiller and my 338-378 Tejas has a Defiance. I REALLY like the Defiance.
Good question. I asked several 'smiths about this last year while planning another custom build. The guys I talked to said they can make a Remmy just as true and it will shoot just as accurate. Cost might be a couple hundred less, it just depends on how you intend to customize the 700. In my case I ended up going with a BAT on my LR rifle, and found an Allen Precision built on a Stiller. Both actions have long enough magazine boxes to accomodate VLD's seated to the lands, and both have M16 type extractors.
Besides, I was building custom rifles and wanted the best components available.
The problem with the remington is that you will have 400 to 450 into the action, another 200+ to have it all trued up. Probably gonna want a new recoil lug. Maybe a new bolt. Frieght...ect...
So you now have anywhere from 700 to 900 dollars into your remmy, and if you ever sell it, it will be worth...$400 to $450!!
And, once you cycle the bolt on a custom...well...the rest is moot!!!
One good thing about a remmy.....every gunsmith on the planet works on them, and the aftermarket is FULL of parts for it , also. I have lots of guys with Brownings, Weatherbys, Sako, ect.... who want to "fix up" thier guns. They are very limmited.
If you don't already own a remmy to donate...go custom...you will NEVER have buyers remorse. I prommis.
I have labored over this very question for the last 2 years. I have a 'donor' action (early 80's Rem 700 from an ADL - my high-school graduation present.....), but not sure if spending the extra $ on a custom would be 'worth it' in the smaller .284 bore.
At the end of my deliberation, I've decided to have my rifle re-done. Initially there is a lower cost involved going this way, but also because i'm planning on staying with a 7mm RM chambering in a med. heavy carry hunting platform (for elk/mule-deer hunting here in CO). I think the stock Rem 700 is capable of excellent accuracy in this application.
However, if i decide to start shooting long distance (in a hunting application) again (=/> 1,000yds), i'll go with a fast .338. For this I believe a custom action will be chosen. Given the extra power in the chambering and distance, i think a custom action (BAT, Defiance or Surgeon) will possess the extra % over a trued/blue-printed Rem 700 particularly with large, higher-pressure chamberings. This is a much more expensive route.....
I have never once regretted the cost of the custom actions I have used.
I have also had a couple of factory actions accurized, and they were really good too. I think probably the least expensive way to a really accurate rifle is to use the factory barreled action, have it all trued, including a trigger job, and bed it into a good stock. Go see the Harvester series at Hill Country Rifles for an example.
I had an old Ruger tang safety in 7 rem mag done this way. I ended up with a rifle that shot in the .4's after load development. I didn't shoot much big game with it, but I did shoot a boat load of prairie dogs.