Hi BH, first off, let me say, once you get a Brit gun, they are like a drug. Your going to get hooked!
If your first strating out, I would heed Dons (Rocketmans) advise. A W&S 700 series is a good safe bet for a first Brit gun. They will have fewer problems than can be found in a vintage gun. That's not to say all 700's are perfect, but if you find one in good condition, it's a safe bet you wont't have to worry about wall thickness, proof etc. But be aware, there are some W&S that were made in Turkey and just had the W&S name stamped on them. So double check to make sure that the gun was made in the UK.
Vintage Brit guns can be a joy, but also a money pit. I would do a lot of homework on vintage guns before buying. There are so many variables and nuances it can be mind blowing. Not complex, just a lot of things that could leave a bitter taste if you get a bad one.
Ask lots of questions and stick with a dealer who will be helpful and willing to guide you. Kirby from Vintage Doubles and Charlie from Hill Rod and Gun are two of the best.
I would also suggest you get Dig Hadoke's book. It is an absolute MUST read.
http://www.amazon.com/Vintage-Guns-Modern-Diggory-Hadoke/dp/160239198XThere are way to many factors to give you a complete answer to your question. Just follow what others have and will say on this post.
Good Luck to you!!! You won't regret getting a fine Brit gun!
Greg