I've had a 'Coast for about 2 years, they are a very odd duck but may be one of the most underrated bikes of all time, very reliable, very nimble and very comfortable in all kinds of riding conditions. My brother rides a BMW R1100RTP and had to admit that he was having a hard time keeping up when we are on the mountain twisties. It's good in traffic and long days on the interstate.
100,000 miles on a 'Coast is common for someone who takes care of it, 200,000 is not unheard of. The '89 had a weak stator but this was fixed on all following years, putting out about 320-340 watts. Seems to me that leaves about 120 watts to run your extra stuff. Calculate what you farkles you want and their current draw and run them accordingly. An aftermarket voltmeter will allow you to keep an eye on your battery.
'Coasts are great bikes, smooth and comfortable, I've had 3 gallons of milk and three bags of groceries in the trunk with room to spare. Change the oil once in while and it will run forever.
There's another 'Coast post from a few days ago you may want to look at as well.