I bought an old one (pre WW2) a few years back at a local gun store. Hard used and not very well taken care of. I got it for $295. I planned to cut the barrel, add the extended magazine tube and refinish it. Once I got it home and started working on the brown surface rust with 3n1 oil and 0000 steel wool it wasn't nearly as bad as it looked.
I decided not to cut the barrel after a gunsmith told me that some A5's came from the factory with poly chokes. There are no starts on the barrel to show whether or not it was a full, modified or improved cylinder choke, so he could be right.
I planned to recut the checkering and refinish the stock so I bought a Ramline synthetic stock and forend for it (and just left it on there).
I got a Nordic extended tube. It holds about 9 or 10 shells. I also bought new springs and friction rings for it.
If you decide to tear it down completely (like I did) you'll needs some special thin bladed screw driver tips. I got mine from Brownells. John Browning had an amazing mind. The internals are really neat on that shotgun. Parts of it reminded me of the M37 Ithaca (which he also designed, but later). You can get parts from
http://www.midwestgunworks.com/page/mgwi/ctgy/browning-auto-5