They are all very reliable.
What do you mean by "investable"? If you mean worth the most, that would probably be the Jubilee model or the P7K3 especially with all the magazines and barrels in a set.
All P7s were made in Germany, none in the U.S.
Many prefer the PSP for carry, but some prefer the M8 because it has the standard mag release most U.S. guns use. Both shoot the same. The P7 went out of production in 2008.