It all depends on your budget, Matthew. Experienced players usually prefer hand built or antique instruments, in which case you're looking at thousands. A 30-button Lachenal, the mid-priced, vintage instrument of choice in former years, will now set you back a couple of grand at least. Several US makers make 'hybrid'(accordion-reeded) instruments for the same kind of price. In the cheaper bracket, the Rochelle has its fans, but you do get what you pay for. Really cheap instruments can be badly built that they're physically difficult to play properly. If possible, try it out first - you already have sufficient experience that you should be able to tell a piece of outright junk.

Go to Concertina.net for discussions of instruments currently on sale. You'll sometimes find people on there discussing Ebay instruments along the lines of "That one looks dodgy!", etc.

Where are you located?