Nice old BUBF. Haven't seen one for awhile. You should be able to get the correct seat on eBay, even if it is not in perfect condition it can be reupholstered.
The carbs will definitely need to be cleaned, and if ethanol was left in it, may need more than that. I would remove the tank, pour out anything that was in it, then pour in a gallon of Superclean, and shake it around. Let it set for a few hours, then shake it again. Do this several times, then pour it out and rinse the tank with water, then pour in about half a gallon of kerosene, shake it good, and pout it out. The kerosene will leave a protective oily coating on the inside. If it is not rusted, or just barely rusted you should be ok. If it is badly rusted, things get a bit more complicated.
You will either need to replace or rebuild the petcock, replace the fuel lines, and probably some of the rubber parts in the carbs. If you haven't turned the engine over yet, I would pour some transmission fluid in the spark plug holes first, and let it soak. If the rings are stuck, and you try to turn it over with dry cylinders, it will do some damage.
If the engine starts and runs, and it seems to shift ok, then I would tear it down, lube the splines, the swing arm bearings, the steering head bearings, change the final drive, engine, and fork oil, lube all the cables, and just give it a good going over in general. With only 6,000 miles on it, that bike should give you years of nearly trouble free use. The old style 1500 is not as trouble prone as the 750, and easier to work on. It's also fairly rare now. 1500 and 1600 Classics are everywhere, but the old 1500 has virtually vanished. While it's fairly rare, it's not a collectors item. If it were mine, I would just put it back together, and ride it as is, leaving the original patina and even the dent in the tank as is. Makes it looks like it has actually been ridden rather than being a trailer queen.
I used to be one of those types that was constantly cleaning and polishing my bikes, now I see cosmetic wear and damage as a badge of honor. You can look at a row of bikes, and tell the ones owned by real riders. They will be the scratched and dirty ones.
I am a motorcyclist, NOT a biker.
1997 Vulcan 750, purchased about a week ago
2006 Sportster 1200 Low
2013 Royal Enfield Bullet 500, converted to carb
2001 Yamaha XT225, heavily modified
2004 Honda Rebel 250
1979 Vespa P200E
2002 Vulcan 750 parts bike
1994 Yamaha XT225 parts bike