Your idea will work great! You should try quite hard to match the front 3 channels. Buying vintage may help you get 3 monoblocks so you don't have to waste an extra channel. The other solution that I would highly recommend is: Get 2 stereo amps that are bridgeable (ie dynaco ST-70) and use 1 for the right and left and the other bridged mono for the center. The center channel is the most demanding anyway and should have the extra power.
However, for the price of a pair of ST-70's you may end up in the price range of some nice solid state 5 channel amps like an Anthem?
However, for the price of a pair of ST-70's you may end up in the price range of some nice solid state 5 channel amps like an Anthem?