Hey there Kww1,   we all need some more info to make any real recommendation. 1)  o7 stumpy- full sus or hardtail?  this makes a major difference in the most important part-the battery and where you mount it. 2)  you looking for a  hub kit or mid drive kit?  do you know the difference?   3)  what are you using it for?  mountain climbing, commuting, or cruising down the bike path. 4) budjet  $.....$$$$      you're going to have to make compromises,  sorry, you can't have it all! 
I have a hub drive kit on an 08 Diamondback response comp.   it was expensive- $2k,  the battery's heavy,   it hauls a** (28mph),  its bullet proof (2500 miles so far),  Its an absolute blast to ride.