I have an HK P9S Target Model that I bought about 20 years ago.
It is far and away my favorite 9mm to shoot.
There is a screw that adjusts how heavy the trigger pull is.
You can dial it down to about 1 lb, but that makes shooting a double under recoil, way too easy. I have mine set to about 2 lbs.
And, as you would expect for a target pistol, it has a fully adjustable rear target sight. It also has an adjustable trigger stop. If you use it, you can't fire the pistol double action though.
I found the grip felt too 'thin' to me, even with my smaller hands, and I found that a Hogue Handall grip sleeve made the gun a much better fit for me.
It is a VERY accurate pistol, with the same 'flat sided' polygonal rifling the P7M8 has.
This means it can shoot cast lead bullets, and it does so VERY accurately. The bore is like a mirror, and there is rarely any leading at all. My loads are quite mild, though: just enough to give 100% functioning.
I can't tell you what a P9S is worth. They are pretty rare creatures, and the value on them just goes up with time, especially for one with the box, papers, and test target.
I think your best bet would be to look through the auction sites, to see if any have sold recently, and try to find one in similar condition.
I have to say that Galco shoulder holsters are pretty pricey now, and one made for the P9S is sure to be long out of production, and if in decent shape, probably worth a bit of money to someone who owns a P9S.
The same for the other holsters, if they are a name brand, and/or well made, and in good shape. You could sell them to offset some of the cost of the P9S, perhaps.
I have never fired a P9S that doesn't have the bells and whistles mine has. The sights would be fixed, and it won't have the adjustable weight for the single action trigger pull, nor the SA trigger stop.
Would it be possible for you to shoot it before you buy it?
It WILL have all the accuracy potential mine has, but if the trigger is too heavy (which seems unlikely on a gun that was so expensive when new) or the sights are off too much (again, unlikely), it might not have the fun factor that I enjoy so much.
This is a fairly rare gun that is sought after by HK collectors, so don't expect to get it 'cheap'.
But, there aren't a LOT of people interested in them, like with the P7's (PSP) and P7M8's, which are smaller and much more pupular pistols.
They are very nice semiauto pistols, and all I've done with mine is clean it, lubricate it, and replace the recoil spring.
I've fired many thousands of rounds of my cast lead bullet reloads through mine, with no trouble of any kind. It feeds, fires, and ejects my many times reloaded, mixed brand, range pick up brass, with no failures, ever.
If you are buying it from a buddy, there is no shipping, and no transfer fees, which should help you out some.