Clean the tank, clean the carbs, clean the air filtes, change the oil and filter, change the coolant, change the rear drive fluid, clean or replace the plugs. Check the tires for dry rot or cracks, replace if needed. Check the brakes. Change the brake fluid, and the clutch fluid.
Seats are available from Mustang, Corbin, Saddleman, Granucci, and Suzuki. None are inexpensive, ranging from $350 to $600. If yours is trash, a used stock seat canusualy be found on EBAY cheap. All years are the same, so anything from 86 up to 2004 will fit. If you are clever you can fit a 2006 or 2006 S50 seat also. I found an '05 S50 seat for $35, made it fit, and removed the sissybar. Looks good.
For good cheap tires, get Dunlop 404s. They handle a ride great, last fairly well. For really great but expensive tires, get Metzeler ME880s. They handle and ride great, wear like iron.
Ronayers.com has all the stock parts, and the online microfiche to see how it all goes together. Cruisercustomizing.com, bikebandit.com, and jpcycles.com have all the aftermarket stuff you could want.
These are great bikes- completely bulleteproof. I've had three of them: an '87 750 I bought very, very used and resold; a 1996 800 that I bought new and put 135,000 miles on (visited all 48 lower states on that bike); and a 1993 I bought with only 7000 miles on it about two years ago and have put 50,000 miles on so far. My Harley is my primary bike, but I always keep an Intruder around, as they are so much fun to play in the montains with. For a cruiser, they are very quick, very agile, and a blast to ride.