I dunno. My old 240SX and Impreza were total rustbuckets and passed safety inspection just fine. A couple more items:
-tr8todd is right about titles: Any car newer than MY1980 MUST have a title, and getting a replacement is next to impossible, even if the car comes from a more permissive state like NH.
-Sales tax in MA is 6.25%. Any car bought out-of-state is subject to this tax, so tax-free NH shopping does not apply here.
-Inspection costs $29. Cash is easiest. If your car fails emissions inspection, you get a sticker with a black "R" on it. You have 60 days to repair the problem and get the car reinspected. You do not need to pay again as long as the car passes within the 60 day period. If your car fails safety, you get a sticker with a red "R" on it. You are not supposed to drive a car with a red R at all unless it is for inspection or repair. That being said, I see plenty of cars with red Rs tooling around, just don't get caught.
-All emissions are read via OBDII. No more sniffers.
-If you decide to buy a different car from a MA dealer or private party, then you are protected by the Lemon Law, which states that anyone who sells you a car must make it safety and emissions compliant. If the car fails inspection, the seller must either repair the fault or give you a refund within 60 days. This does not apply to cars bought out-of-state, so buyer beware.