A Heckler & Koch PSG-1 or a mint-in-hardcase original Russian Dragunov SVD, either one cheap for about $12K
A flamethrower!
Serious answer: the Walther PPK, just for the James Bond reference.
Are you sure it’s a Mauser K98 and not an M48 or M48A? There’s very little difference between them mechanically, (the M48/As are very well made, easily as good as the K98s) but an M48/A is worth about AUD$450, whilst a genuine WWII K98k is worth about AUD$1000 if it has all the original markings, including the swastika, on it.
silenus, everybody and their brother did seem to issue a Mauser or derivative thereof at some point, with three notable exceptions: The British Empire/Commonwealth (Lee-Enfields), the Russians (Mosin-Nagants), and the French (Lebel M1889/MAS-36).
Funnily enough, the Lee-Enfield is still in active service in India and Africa. 
I’d love a H&K USP Mark 23. The ultimate big-ass .45 hand cannon. Plus if it’s good enough for the Navy SEALS, it’s good enough for me. Oh, and throw in a supressor while you’re at it.
Not sure I want to spend $2,400 on a handgun though.
Well, the semi-auto version is only worth a couple thousand, I believe. Class III firearm prices really took off in the past 20 years, since only pre-1986 guns are available.
Here are a few for sale, and some class III info, too.
The Holy Grail for handguns is the SIG P210. The only way it could be better is if it came in heavier calibers (I’m not a 9mm fan) but it’s hard to improve upon perfection.
Rifle-wise, I’ve always wanted a bullpup. I’d take a Steyr AUG, an IMI Tavor, or the aforementioned Walther WA-2000 without thinking twice.
I’d also get a .50AE Desert Eagle simply for the novelty of it and I’d also get a Cobray Streetsweeper, a GLOCK 18, and (my personal favorite) a Serbu Super Shorty.
I’m thinking I got’s me one of them!
For me it would have to be a British L1A1 preferably one that took part in the Falklands War.
I’d love to have a nicely engraved British double-barreled express rifle. The kind that are custom-fitted to the customer in a fancy shop in London, and if you have to ask the price, you can’t afford it.
Let’s see…
I’ll take a set of matched Holland and Holland Royal Double Sidelocks, one in .700 H&H and one in .375 H&H, just to hang over the fireplace.
A damascus barrelled Parker Shotgun (with hammers please) would be nice.
In the functional side of things, I’d like a Seecamp .25 auto, a Les Baer custom .45 1911 varient, a Rock River Arms AR varient, an FN/FAL, a Steyer AUG and an H&K MP5. Throw in a Browning BAR and a Steyer Scout and I’m pretty much good to go.
I fired one of those once.
I want one.
I’ve got a double-barrelled muzzle-loading percussion 12ga shotgun made in the mid-19th century by I. Hollis & Sons of London.
Nothing adds a touch of elegance and class to a room like a 19th century British muzzleloading shotgun, IMO. 
Except of course the aforementioned British bespoke big game rifles, with the requisite elephant and water buffalo engravings and platings.
Oh lord yes! £100,000 H&H double… I’ll be in my bunk.
These seem so… so vulgar -
M1 Garand, M1A National Match, M25 White Feather.
I’d be very happy with the relatively-cheap (when compared to an eighty thousand dollar gun, at least) Sig Sauer SSG 3000. It’s just so damn sweet.
Handgun-wise, my current wishes are a Springfield 1911, and a Walther P99.