If the defender buys the Bunker Complex which includes the 8FP mines, and during the game plays hidden minefield, what strength that new minefield will be?
There will only be one strength of minefield in a game. So if the Bunker Complex w/ 8FP Mines was purchased, the Hidden Mines action will place 8FP Mines on the map.
Notice that there is a Mines counter w/ 7FP on the front /8FP on the back that can be placed on the Mines box on the track display to remind players of the minefield strength in the current game.
So to answer your second question, purchasing the Bunker Complex includes 8FP mines. The defender could not then purchase a Hasty or Prepared Minefield as the strength doesn't match.