Worth, I'm no expert on the market, but I do know some things about collecting, and tradeing of double rifles. If you will keep my amateur status in mind, I will try to answer some of your questions.
#1
You ask about new vrs old double rifles, and I will just say there are new rifles, and then there are other new rifles. Of the two you mention, the Searcy will appreciate faster, and to a higher price than the Merkel, IMO. The Searcys' are fast becomeing the choice of PHs who buy New double rifles. Once a lot of hunters find out how good the Searcys really are, this rifle will take wings, again, IMO. If this continues, the value of used Searcy doubles will hold there own, and appreciate when the rifles are wanted by clients who are in a hurry to get one. Right now the Merkel actually looses some value on the re-sale, but that will change as well. It will just take longer than the Searcy. On the new rifle market a new Searcy is a sleeper, and a real bargin.
#2
A referbished double rifle is nothing to worry about as long as it is done properly. IMO, a real selling point is a rifle that has been referbished to "as new" by the maker. There are many things to look for, and some are more important than others. one thing that some people do is, pean the surfaces of the hook, and pin to make the rifle seem tight, but this does not last, and is an unethical practice. A restocked double rifle is something that is, if done properly, OK, but if not, can really effect value. Thankfully this is easy to spot, and if it is done right, the overall quality of the work will be evident. It does effect the value, however.
#3
Right here let me say any rifle chambered for 470 NE is automatically worth $1000 more than another rifle of the same make, and quality. This is not to say that the 470 NE is any better than the others in the same class, but that it is the most popular right now, and you will pay a premium. The standards are OK because of their ammo availability, like the 450 NE 3 1/4", 500/450, 500/465 are all good chamberings for a DGR, and are cheaper than the same rifle in 470 NE. The one thing you need do here is make sure the rifle is NITRO PROOFED, because some of these were offered in balck powder, and then NITRO FOR BALCK, and "EXPRESS TRAIN" models that were regulated with light bullets. NEVER buy a rifle for an OBSOLETE cartridge, unless you know, absolutely, that componants are available.
One real sleeper is a quality rifle chambered for 450/400 NE 3", but folks are getting wise fast. The 3" version was never offered in a black powder version, but the 3 1/4" version was, and some of the rifles are not NITRO PROOFED, in this length. Any "GOOD" dealer will make you aware of the proofing.
#4
The prices on any of the better dealer websites will reflect the going prices at any given time. With Name rifles they will appreciate even when bout at today's prices.
#5
www.champlinarms.com
www.hallowellco.com
www.doubleguns.com
www.searcyent.com
www.gunsamerica.com
www.capeoutfitters.com
The above are a few of the daelers who have double rifles in stock most of the time, both new and used.
#6
I would dearly love to find a 450 NE 3 1/4" Westley Richards best grade double rifle in the maker's oak and leather case, with all the tools, and factory snap caps
A couple books you might think about aquireing are: Alaxander Gray's book
"THE HAMMERLESS DOUBLE RIFLE" mostly H&H rifles but this book has some very valuable information in it. Another one is Graeme Wrights books, #1, and #2 "SHOOTING THE BRITISH DOUBLE RIFLE" . There is quite a bit of good info in Boddingtons SAFARI RIFLES, as well.
I hope any of this helps, and you can bet that on this forum there will be plenty more folks who will tell you some of the things I forgot! 
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..Mac >>>===(x)===>
DUGABOY DESIGNS
Collector/trader of fine double rifles, and African wildlife art
