Thanks for the suggestions. I like the look of the Husqvarna 51.
Is there any rule of thumb to determine whether a particular hammer gun will handle modern loads?
You need to ask a lot of questions before you buy. They are no different than when you purchase any older sxs. You should always ask about the hammers. Make sure they are not loose, that the hammers and their screws match. Check the function of the hammers when you get the gun. Get a 3 day inspection period. Have a good smith give it a going over. In particular the barrel wall thickness should be measured at this time. Not just the chokes but the full length of the barrels.
The 51 had a long history of manufacture. Look for one made after WWII.
If you find a gun, get the serial number and go here to date it:
http://home.wanadoo.nl/~fdwjr/SerialNr/YoM.html Here is my restored M51 12ga.

You can try:
http://www.sarcoinc.com/husqvarnas.html Make sure you ask a lot of questions. Some of the guns here are in need of a little fixing up. At least that was my experience with them. Although they were very forth coming with information.
Also:
http://www.kebcollc.com/ Although their website does not show any at the moment. Give them a call. I have dealt with them and found them to be honest.
Also check on GunsAmerica. There seem to be a number of dealers there with a large supply of Husqvarna shotguns. Be prepared to see a lot of Lefaucheux actions.
This forum is devoted to all guns Husqvarna
http://www.gunboards.com/forums/forum.asp?FORUM_ID=41The Husqvarna is a good hammergun for your 1st. It will be a reliable shooter that you do not have to worry about. If you decide to open the chokes you will not worry about "ruining" the gun's value. I use mine with heavy pheasant loads (Fiocchi Golden Pheasant, 1-3/8 ounce) with no problem. I would have no worry using 75mm bismuth duck loads with it.
Pete