If your considering a used firearm look for something that uses the 209 primer, or can be converted to use it. They are more reliable than the 211 in wet weather. Make sure the ammo is available, some manufactures such as Gonic had specific tolerances that make it hard to find ammo.
There are some very nice mzls available today, my personal favorite is Knight
http://www.knightrifles.com/
50 cals are about the most popular today so there are alot of accessories available for them.