DoubleA,
I'll give this one a try, and I'm sure that others will provide input as well.
1) The "new Contender" that you speak of is a G2 Contender. Slightly heavier than the old Contender, different grip design than the old one too. Most old contender barrels will fit the G2's frame. The Encore is heavier than both the G2 and old Contender and is designed for higher pressure loads (the old contender and G2 are designed for lower pressure cartridges and loads). Barrels are not interchangeable between the Encore and the G2 and old Contenders.
2) I believe that all frames are capable of being converted from pistol to rifle/carbine and back, however there have been many different threads and postings dealing with the legality of this issue. I am not certain as to how purchasing a frame only would be. I would assume that the frame would have to be listed as either a pistol or rifle, and according to what others have stated, may not be converted from pistol to rifle unless proper documentation and precautions are taken to meet Federal Law. I am sure that others have a great deal more to say on the subject...I'm not an expert on the legality issue.
Whatever you choose to purchase (G2 or Encore, pistol or rifle/carbine), you're sure to have a fine firearm in your possession that is capable of nearly endless combinations.
Hope that this helps.