I had a great, almost hour-long chat with the gunsmith at my LGS. Really good guy, knowledgeable and helpful! (they even have videos on YouTube of him doing a rifle build - fun to watch and learned a lot)
It sounds like it'd be around $600 to do all the work to assemble and true the gun if I bought the parts. So, my updated parts list is:
Origin action - $900
SS Proof blank M24 profile - $449
HS Precision PST006 - $669
TriggerTech Diamond - $299
-------------------------------------
Total: $2317
Total + Build: $2917
He really stressed how important the barrel is and said if I were to get a used rifle, the first thing he'd essentially do is get a new barrel for it. So, if I get, for instance, this 700 5R Gen 2 on Gunbroker, that'd be another $449 for the blank, and the cost to fit it to the action, etc. So I'd still be looking at about $2000-ish for him to work over it. About 3-4 week turnaround for him to assemble the parts for me.
The nice thing about buying the parts is I can do them one at a time, which makes the whole price of $3k a lot easier to swallow.
I'm really leaning towards the custom build now.