Setups usually are not that pricey. If the truss rod will bring the neck level (or nearly so), and there's room to shave some height off the saddle, then you'll probably be ok, bearing in mind that you can't run as low a saddle on a 12 as you can on a 6 (because of the greater distance from the saddle to the pins for 6 of the strings). That being said, Fender acoustics are not generally known for their durability, especially the 12s. If you can get the neck straight, set the edge of a yardstick alongside the fingerboard from the nut to the bridge. The end of the yardstick should be about level with the top of the bridge. If it hits the bridge much below the top, probably no amount of adjustment will restore playability short of resetting the neck, which would likely cost more than the guitar is really worth. Hope thsi info helps.
Sandy