With that kind of budget you can get a pretty nice set up. Are you looking for a single barrel, a pump action, or a semi auto?
What kind of distance are you looking to get out of a slug gun? If most of your shooting is within 0-60yds I would just use a smoothbore with rifled slugs or a rifled choke with sabots. If you think you may need some distance I would look for a true rifled barrel and use sabot slugs.
What two purpose's are you talking about when you say dual purpose. There really are tons of combinations, between barrel lengths, smooth vs rifled, sights/optics, chokes and load selection. To be honest any standard barrel with screw in chokes can fufil most needs, there are better combinations for each purpose but you could get by with just the right screw in choke, and proper load selection.
You know when you do mount a scope that's going to do away with it's dual purpose unless you're talking about a deer/turkey gun or plan on removing the scope everytime you put a different barrel on. Even then you will really need to dial in your scope every time you change barrels or loads.
If you really think you need a scope I would suggest buying the shotgun of your choice, semi, pump, whatever brand you prefer and also buying a rifled cantilever barrel to go along with it too. That way you could switch out to the longer vent rib barrel for whatever hunting and the rifled barrel and scope would stay together and zeroed.
If I
wanted to spend $1000 I would probably go with a semi auto, like a Browning Silver, Remington 1100/1187, Beretta 391, Benelli m2, Winchester SX3, ect.
If I
could only spend $1000 I would probaly go with a nice pump. These come to mind, Remington 870 Wingmaster, Browning BPS, Ithaca 37 Deerslayer.
If you wanted to spend $500 or less on a reliable but not as pretty shotgun then you could with a express model 870, Mossberg 500, Winchester 1300 or a few others that will get the job done just fine.
Or you may even find a decent combo somewhere. Something like these.
http://www.budsgunshop.com/catalog/product_info.php/products_id/42056
Or these if your not too hung up on having a scope.
http://www.budsgunshop.com/catalog/advanced_cat_search.php?cated=48&ITATR_list[3]=9145
