Wow. I know so much about this it's not even funny.
The "guy in Birmingham" is Bob Tedrow. From all accounts he has an extremely extensive collection. And he does make his own. From all accounts, they're great.
But first a little anatomy lesson. There are actually two types of concertinas (well, 2 main types... there are several others): English and Anglo. The main way to distinguish them is that on the English, a button will play the same note regardless of whether you're pushing the bellows in or pulling them out. On the Anglo, a button plays a different note on the push and pull. There are several theories/beliefs on which one you should learn to play. In general, it seems like people who want to play a lot of Irish music go for the Anglo, because the constant in/out motion of the bellows gives your music an intrinsic "lilt". The English has more buttons, and is reportedly a little easier to get the hang of. Bob Tedrow's concertinas are Anglos, and this is what I play as well. I will say though, that when I finally tracked down some local Irish concertina players, they all played the English. And since you play them completely differently, they weren't of much help to me.
I only know about the Anglo, so I'll talk about that. First off, Anglos in general come in two varieties: 20 button and 30 button. 20 buttons are cheaper, but you're limited in what notes you can play. I'd suggest going ahead and starting with a 30 button, since it's a move that will eventually be made anyways. 40 buttons are available as well, but lots of folks never feel the need to buy one.
As for price, it sort of breaks down into 3 categories: beginners, mid-range, and god-like. A quick look at some classifieds show the range of prices to be from 550 dollars to 4500 dollars for used concertinas, so that requires some explanation.
For some strange reason, there's a prevailing opinion that the best concertinas where the ones made shortly after the instrument was invented, which was in the mid 1800s. And rather than being museum pieces, people still play them. Brands like Wheatstone, Jeffries, and Lachenal are examples of these. A good 30 button from this era can run anywhere from 3000 dollars on up... for a 100+ year old instrument. There are also some modern day makers that command almost as much respect, and their stuff cost almost as much money. However, unless you find their instruments used, you'll be wating several years to get ahold of one.
In the mid-range, you've got makers such as Bob Tedrow, Harrington, The Button Box, and Morse. Their concertinas all run anwhere from 1500 to 2500 dollars. In general, they're of top-knotch quality. The difference between them and the more expensive one is that they're a little less fancy, their response time is a little slower, and they used accordion reeds (rather than concertina reeds). These are differences you wouldn't notice as a beginner.
In the cheap/beginner market, you've pretty much got one option. A company called Stagi mass-produces concertinas. They're actually serviceable and pretty. That's what I play, and as a beginner, I'm happy with it. New they can run from 550 to 750. I picked mine up used for 350. But also, Bob Tedrow sells a modified Stagi. He takes the stock model, and gives it better action, new buttons, and in general makes it a much nicer instrument. These sell for about 850 dollars, I think. This is what I'd probably buy now. There's a used one for sale on concertina.net for 550 dollars right now.
Yikes. Far too much information. I don't think you live in Austin, but if you do, I'd be happy to let your boss play around with my concertina, to see if it's what he expects.
Finally, here are some useful links:http://www.concertina.net/buysell.html
Some prominent players/makers hang out here. They've also got a Buy/Sell section where you can see the current used instruments for sale. And a ton of other useful information as well.http://hmi.homewood.net/
Bob Tedrow's site.