Definitely take a look at the web page referenced above.
There is a resource on that web page that will let you look up the serial number and determine the year of manufacture.
$550.00 is between ok and the high end of a stock Hi Power. You can get a brand new one for about $595.00. You should make sure the gun is in like new condition for that much money. I recently picked up an unfired 68 Hi Power in absolutely flawless condition for $550.00
Some will ask what I think are ridiculous prices. I have seen stock, older high powers that the owners want $750.00 or more for. If you are not a collector or a real purist then that is not a reasonable price.
If you want a shooter, try to keep it around $450.00 to $550.00.
Of course there are always deals that you hear about. Like the guy who picks one up for $300.00. If you come across one of those and don't buy it, please let me know .
I'm still looking for that deal.
But it is an excellent pistol, an absolute classic, and a real delight to shoot. So if it fits your hand then you will probably find the gun a real joy to shoot at the range.