I'm sorry to hear you have had such bad luck with theft. People do like to lift guitars... a warning to all of us that we never can be too careful.
Here's what I know about Martins.
I have owned a few in various sizes: 0, 000, M and D. I think the M is a lovely compromise for most folk music although it may not be ideal for any. It can be fingerpicked, flatpicked and strummed very well. The size is handy too being roughly in-between a 000 and a D. The volume is adequate for almost any purpose. If I could have only 1, I would have an M. This is a personal choice, however.
You hear a lot of talk about rosewood. I have had a couple of nice rosewood Martins (my M is rosewood) but I have had some very good mahagony ones as well. I recently sold a 1952 0-18 that both Andy O'Brien and Mick Moloney played on my CD. They loved it. It was a gem. My musical partner, Bob Conroy, plays a practically new Vintage D-18 that is very loud and very evenly toned. I have heard quite a few Vintage D-18s and they have all been great guitars. It is the bracing, I guess, that makes them superior to standard D-18s.
I have had some dogs too. Guitars that looked great and never matured in terms of sound. The difference in sound quality between individual guitars of the same model can be trememdous. I would say to anyone considering a Martin to only buy a guitar that sounds good now; if it improves with age, so much the better.
A few words of warning and reasons to consider a newer Martin rather than an older one. Pickguard cracks: Martin changed the way their pickguards are fixed to the guitar top within the last 15 or so years. Cracks in the guitar top caused by the pickguard shrinking were far-too-common in instruments made earlier. Adjustable neck rods: Martin started placing adjustable neck rods in their guitars in 1985. An adjustable neck rod can save you the $350+ cost of a neck reset many times over.
The workhorse of my stable is a plywood Sigma DM-4M that has passable sound, good action and on a value-for-money basis is one of the better new guitars to be had. Having said that, I think the "stripped down" an all-mahogany Martin D-15s at about $550 discount retail is good value.
All the best,