I'll add my voice to the choir on this -- look into a neck reset, along with whatever additional repairs might be needed in the bridge area. Bridges sometimes pull away from the face, but can be reglued. I'm not a huge fan of Guilds (personal preference), but a D-18 with some miles on it can be a damn nice guitar. There's something about the character of an older, well-played Martin dreadnought that can really create some magic.

Once your guitar has been taken care of structurally, it should handle medium gauge strings with no problem at all. You also should have no problem shifting into alternate tunings, particularly those that are reached by loosening some of the strings -- open D, open G, DADGAD, open C (one of the strings is raised a half-step in that one, but it's no big deal), etc. If you play with very low action in standard tuning, you may need to raise it a bit to a "compromise" point to adapt to slack tunings. Other than that, your guitar should handle it just fine -- my guitars always do.

UB Ed, I also have a Lowden O-style, and I can't play it enough. Great guitars.