I was just where you are a few months ago (have wanted a humvee since I got out of the service many moons ago, but just got serious this year). The advice posted so far is good, especially thinking about what you want to do with it and investigating registration. For me, I plan to just drive around the little town where I live a couple times a week. No highway rides for me (I well remember being passed like we were sitting still on the Autobahn--not fun) and definitely not a daily driver/grocery getter. So I bought an older M998 in reasonable condition in the configuration I wanted (four-man soft top) that was already titled.
Do think carefully about and fully research titling in Mass. One thing I was concerned about in my search was buying a truck and then having titling/registration problems. Apparently even conservative Kansas, home to a ridiculous number of veterans, can be picky. So do your due diligence and if possible talk to other Mass folks about your issues. Two different guys from Kansas pointed out a few pitfalls to me that I was able to avoid based on their advice. We're not supposed to get into specifics on the forums, so hopefully you'll get some helpful PMs as TOBASH said.
One final thing, which is true of all car buying endeavours: don't be in too much of a hurry. Decide what you want, how much you will pay, and then stick to it. Don't settle for a two-man engineless beater because the money is burning a hole in your pocket. I talked to a whole lot of sellers before settling on the truck I bought, and I think the patience paid off.
Welcome to the forum, and good luck in your search.