So I'm changing up my acoustic guitar, for years i've played a martin 000-15ce that I've loved, but it's time to move on. After playing a lot of different guitars from different builders, I seem to like the sound and playability of the Martin D-18 the best. The thing is, there seems to be a wide variation into what D-18 means. Generally it's a martin dreadnaught with a mahogany neck and body with a spruce top. there seem to be subtle differences depending on the year and there are also several models: retro, golden era, authentic, etc... and a wide variation on cost/value. It seems a lot like buying a strat, there are many different flavors of strat out there. What are the differences in the various models? anything i should be on the lookout for? Hopefully this will be the last acoustic i purchase, I'm selling several guitars and saving up to make the right purchase. I would like to buy something that holds it's value or increases in value over time, but i have no plans to sell it ever. Probably looking for something used in good condition with no structural cracks. If you have any advice or knowledge I'd be happy to hear it.