first, I would strongly suggest you not buy a used double as your first double rifle. ive gone threw hell dealing with used doubles and have pretty much sworn off doubles because of it.
second, I would recommend picking a caliber that will do Dangerous game so your not restricting the rifles use. 450/400 NE 3" and 375 flanged are good options but ammo isn't common for either cartridge. 470 NE is easily the most common cartridge for doubles and ammunition is abundant. these days people have been very successful with rimless cartridges in doubles such as 458 WM and 375 H&H but there are also serious issues that can come from not having a rim. if your thinking about a rimless cartridge make sure to thoroughly research the topic before making the plunge.
if you never plan on taking DG (unlikely if you hang around this forum) then the 9.3x74R is a great choice. the problem comes when you decide you want DG, the 9.3x74R isn't legal to use for DG in all African countries so if you wanted to use your double you would need to pick a country that allowed it.
if you really liked the Chapuis double in 470 NE then you should get one. even if you never go to Africa I guarantee you will find a use for your 470 NE double.
-matt
I am +1 with matt85 here, in that if you can afford a new double, it will likely have some type of warranty in case it is faulty.
And regardless of the fact that you may have just spent 10 grand or more on your new rifle, there is no guarantee that it will arrive to you in perfect condition.
So, the warranty is mighty comforting in that unlikely/unpleasant event.
The following is a case in point;
My worst double of the several I have owned was a brand new Merkel SxS, which I had ordered from the US importer/distributer, when I was working part time for Great Northern Guns, in Anchorage Alaska (about 11 or 12 years ago).
The right side chamber or throat was tight enough that the factory ammunition (Federal 300 gr round nose) it was made for (according to the paper work that Merkel sent with it) showed high pressure symptoms.
The left side chamber was perfect (and the left side shot as tight a group as one might expect from a bolt gun in the same caliber).
The right side chamber pressure signs included very flat primers and sometimes one would rupture, causing primer metal to poke back through the firing pin hole, making it difficult to open the rifle.
Plus, the bum barrel flung it's shots somewhat vertically, usually three shots into about a 5" string most days at the range.
I decided not to send it back to Merkel but instead to have JJ Perodeau re-regulate the rifle to my hand load of 300 gr round nosers at 2400 fps, at least partly because that is my favorite load in that caliber for bolt guns and single shots any way.
So, I wished to have all my ammunition the same for both my Mauser and my double.
Otherwise, I presume Merkel would have fixed their error or replaced the rifle with no charge to me.
This expert work by Mr. P. totally fixed the problem and I happily paid for it, along with him installing claw mounts and a Leupold 2.5x scope.
Considering what he rightfully charges for claw mounts, he hardly charged me anything for re-regulating to my load and zeroing my scope to 100 yds for me.
Also, my already acceptable groups when firing from the sticks with the classic "old right and left/soft and solid" shrank even more, thanks to Mr. Perodeau's expert work.
So, although buying a new rifle will ensure that you have some recourse in the event that the product is faulty, it is no guarantee that it will be right when you receive it new in the box.
The Merkel still seems like a bargain to me as double rifles go but they only seem to be made with the infernally short 23.6" barrels.
If only they would offer their 141 Model SxS like I had but with ejectors and 26" barrels, I would have bought one in 450/400 by now and perhaps that would have ended my constant trying to figure out the best double for me without breaking the bank.
For all that, I think I would look strongly at Chapuis and Verney-Carron next time in the 10 thousand dollar range.
If I could afford a somewhat higher price, I definitely believe the Heym PH Model or the older design but quite similar Model 88B would be extra money well spent.
Heym Trumps them all in the under 20 thousand dollar range but finding one brand new for about the same price as those three afore mentioned makers is not likely to happen (especially in the super desirable 450/400 caliber).
Out.
My 2 bits which are worth less than that, .450/400 
Do not sell Heym short as they build quality rifles. 
