The most important thing to look for is fit. if it doesn't fit you, stop right there. Personally, I have never come across a gun with an English straight stock that fit me.
Second, is mechanical condition. Does it lock up nice and tight? Is it on face? If it a used gun, ask to shoot it. Are the ejectors timed properly?
Third, cosmetic condition. Has it been refinished? Any rust, Any cracks in the wood? Re-blued?
Last is price. Is it within your budget? Sometimes it is wise to stretch a bit in order to acquire really fine gun.
By example, a friend of mine found an L.C. Smith .410 sidelock gun built in 1920 at Kevin's in Thomasville GA. It fit him perfectly. Kevin wanted $10K for it and wouldn't budge, but he did say he would accept my friend's check. My friend wrote the check and took it home. He is absolutely deadly with it on quail. I have seen him make clean kills on quail at 40-45 yards. He wouldn't part with it even if offered twice what he paid. For him, it's the perfect shotgun.
