Overall, you find amazing consistency with vintage Martin ukuleles. What I mean more specifically is that they are consistently good. Luckily, Martin made a lot of them and they often survive quite well. It is helpful to hold one in your hands and see if it speaks to you but that isn't always an option. I have A LOT of vintage ukuleles and I have often bought them from far off lands, and I have yet to be disappointed with one. However, I am not looking for a specific thing in a uke and I don't expect all ukuleles to sound like modern tenor ukes (which I find most people expect). Vintage soprano ukuleles from Martin tend to be bright and punchy sounding and not "guitar-like" at all. I only mention that because I find most people expect a deep bassy guitar like sound which vintage ukuleles were never intended to have.
Places like Elderly and Grphyon are excellent and you will pay a premium but you will likely get an excellent instrument. However, keep your eyes open out there. I see completed auctions on eBay all the time that I think were steals for the buyer. Of course, everyone is onto Martin so you don't see that often with Martin ukes but you do see it with others that are as good or IMHO even better. Good luck!