Here's a recap of custom actions: Custom Actions
... It's a little dated, but a good start nevertheless.
You'll be hard pressed to find a 591 for under a grand. They're above $1400 now at Brownells.
Surgeon makes their Short Round (RSR) Actions. They're under a grand and very good, but you'll need to add the rail which is another $145 or so.
Look around the site for Lawton feedback.
actions also look to be a good alternative to a blueprinted 700.