Most of the time, Contender and G2 barrels will interchange, even if you have to polish the locking lugs on the barrel (see another post about this).
The pistol grip will not interchange with an original Contender and a G2.
The forearms from an original Contender may have to be altered slightly to make them work on a G2. The screw holes will usually line up fine, but the "ears" that cover the hinge pin will need to be shortened so that the won't rub against the reinforcing straps on the G2. The factory forearms for the G2 have a cutout in those ears that allow the forearm to miss the reinforced area of the frame.
The Encore is basically a completely different firearm that just looks like a G2. It's bigger, designed for larger calibers. Nothing from the Encore will interchange with the old Contenders and G2's. You can get chamberings for the Encore that are the same as the Contender, but they are mostly from custom barrel manufacturers.
Whether you choose a Contender (G2 or Original) or an Encore depends on what calibers you ultimately want. I prefer the Contender as a handgun and the Encore as a rifle.
