Walther P99. Buy the one w/o night sights. Then check out the night sights available for it and see which ones you like best. Its the best 9mm out there in my opinion.
If you're a fan of steel handguns, how about a Browning Hi-Power or a CZ-75? Both are SA designs which have a great "feel" to them, much like the M1911.
That's kinda what I'm saving for right now unless I change my mind. I'm about half way there with no guns I want to trade or sell and money is tight so I have time to decide on something else. I just want another .45!!How about the XDm in .45 ACP? That's about as cool as it gets.
Upon seeing the full list, I like the .327 sp101/taurus subby idea or maybe an older colt revolver (for variety).
I would get a mosin and 5 crates of ammo.
It is a piece of history, and they're going to go up in value, just like Lugers did. It will likely be a good, if not great shooter. But the real value is historical. It will be a true collector's piece.
Speaking of which, do you have a Luger? If you've got 40 guns, maybe you should sell one, add the proceeds to your account to have $1k, and get a Luger, before they go up any more.
I shot one a few weeks ago, and it was wonderful. Perfect balance, very accurate, very low recoil. These too, are only going up.