One should look for a Searcy double... at least that's what I heard. They would be my first choice if I got one. www.searcyent.com
Sevens

That's a big question. Double rifles come in cals from 22 Hornet to 8 bore, and price from $1,500 to $50,000 and more, very old (1890s) or very new.

Here are some considerations:
1. O/U or SxS.
2. Small and light (for plains game) or big and heavy (for dangerous game)
3. Blackpowder ctg or nitro
4. Ctg commercially available eg 470 nitro, or reloading proposition only eg any BPE.
5. Working gun or showpiece.
6. Boxlock (stronger stock, weaker metal) or sidelock (vice versa).
7. Ready to shoot or needs work.

So there are lots of dimensions to consider. Sort of like asking "should I buy a bolt action rifle". Here's my general opinion: doubles are ideally applied as stopper rifles for nasty beasts. They should be of heavy caliber, at least 375 Flanged Magnum or 9.3x74R, and the gun should weigh 9 lb plus. The ctg should be rimmed. The gun should have double triggers, a straight stock, and wide V sights. It should fit you so you can shoot more or less instinctively. Ejectors better. If you don't need to reload, the noise is not a problem. If you do, you will need to load fast, and there will be plenty of noise anyway. Top lever is faster than Jones underlever. Hammers are fine, no need to pay more for hammerless.

Avoid cracked stocks and guns that are off the face at any price. A cracked buttstock is a $3K fix. Loose ribs and pitted bores are OK if the price is right. Ribs can be resoldered, but reregulation might be needed, figure $0.5K to $1K. Pitted bores are acceptable, we are talking minute of grapefruit here and most guns with slight to moderate pitting still shoot very well. Besides, anything older than 50 years will likely have some pitting.

Here are two ends of the spectrum: Valmet 412 or 512 O/U "System", in 9.3x74R with a set of 12 ga bbls to make it a one gun battery. Under $2.5k. Holland and Holland Royal Sidelock Ejector, any NE, over $35K.

Don't buy a medium caliber (the 450s) and expect to kill buffalo with it. With the older ctg, serious stopping power starts at 577. With nitro, starts at 375 flanged.

Get a good double gun guy (Ken Owen or Keith Kearcher) to look at the gun before you buy...doubles are expensive to fix, as parts have to be made by hand in most cases.

I think Russ means "don't look for a medium black powder bore (.450) and expect to kill buffalo with it." The .450 NE is the king of double calibers. Otherwise, do what the man says . . . or go see Butch, go see Butch, go see Butch.

That's exactly what I meant. A black powder 450 anything is like a black powder 45-70...all bark and not much bite.

...there have been numerous threads on nitroexpress.com discussing non-negotiable features that a double rifle should have...one features that really figures prominently is the need to have the dr chambered for rimmed cartridges such as the 9.3 and any of the nitroexpresses...