My personal opinion on 700 vs custom...
If you already own the 700 action(s) or got them really cheap, then go ahead and build off of it. The machining and truing costs aren't much and you'll be in about 1/2 of what a custom action costs.
But, if you don't have a 700 action, and you want an action that's already machined and needs no truing, and is ready to thread a barrel on, and you have the money to drop on it, go with a custom, like a Defiance Deviant action.
I just happen to always have a spare factory action, or a rifle I bought cheap used in a caliber I don't care for, or a burnt-out barrel on one, so I just use them to build off of.
Browning A-Bolt actions are great custom actions, too...If you can find a smith that can work them. I'm fixing to have one built on my A-Bolt II once I get the cash for a new barrel.