Check out the "World of HK" link at the top of the page. It has brief descriptions and photos of each model. All are excellent weapons. You can't go wrong with any of them.
The P7 or PSP is the least expensive. The M8 is a product improvement. The M13 is a double stack magazine version of the M8 and thus had a wider and longer grip. The M10 is the .40 S&W version. It was only offered for a brief period, usually pulls in big money, but is very heavy and awkward. Many people comment on this being their least favorite of the line. The M7 is the .45 version and never made it beyond the prototype phase. Very cool though, if you ever get a chance to hold one. The last model is the K3, which was offered in several smaller calibers. Models which come with all calibers are seen as collector models.
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