The one thing I would advise is not to let any seller convince you the gun is worth $600 or $700 or more!
Since I picked up my first one, after looking for about six months, I have found them to be all over the map for pricing. I picked up a never been fired 1968 HP for $550.00 (actually traded for 300 cash and a gun I paid 250 for). Not many people know a lot about them and so they carry this unwarranted bias toward a higher price.
Unless you are collecting or attempting to purchase one that is special for some reason then $550.00 is about tops for a perfect one. You can buy a brand new one for that much and the older ones are, arguably, the same gun with less durable (though still excellent if Belgian) metal.
You will see them, frequently, for much more. This is non sense. They are not rare. Don't buy that malarkey. I had a fellow wanting to sell me a beat up MK I for $500.00. When I checked the serial number it turned out to be a surplus Canadian military pistol manufactured in 50 or 51. Absolutely nothing special and not worth more that $250 to $275.
They are a great pistol, however, and you will love to shoot it because they do handle nice and are fairly accurate.
Take the previous posters advice and check the serial numbers etc over at the fnhipower page. There is a wealth of knowledge there and a lot of friendly people that really know the details and ins and outs of the pistols.