Looks to me like you might get some time with it before major work, but not a lot. It isn't good by any means. If you mean the neck is straight regarding relief, or a slight bowing, that is not the major factor regarding action. Much more important is the straightedge to bridge, which in your photo contacts the bridge well below the top of the bridge. It should, for optimal action, be at or slightly over the bridge. If the bridge has been shaved, a too common occurrence, that's too bad, as it is likely Brazilian rosewood. So worst case, you'd need a neck reset, new bridge, saddle and nut, and probably a refret. It is not the guitar to pay more than $800 for. Add a thousand or so to the purchase price. This all without an in hand inspection, so plus one for a pro to look at it.
Worth fixing? An emphatic yes from me. I did just this with my D 35 and I'm enjoying it nine years later. I had a total of $1400 in it when market worth was $900. I wouldn't sell the guitar for that. Some sellers will never be realistic and some buyers will be ignorant. That D 25 looks like a flatback all mahogany, a rare but not exotically rare bird. Other things that I'd look to see are original tuners, three on a plate, if only to verify someone didn't drill the headstock. They aren't great, but they work. It's a guitar I'd want for sure, and preservationist that I am, would slightly, but not exorbitantly, pay more for. All in for $1800 is something I'd do, but I daresay most, won't. All in for $1400, I'd write the check, not that people do that anymore.
