how much coolant should I buy?

[AlaskaWing]

I was going to dump the radiator and replace the coolant, but I was wondering how much is enough?

GL1100 '82

Is there anything I should be looking for as I change this out?

 

[backlander]

System should hold around 3.6 quarts so 1 gallon of 50/50/ premix should fix you up. Watch out for rust, general filth and the infamous OIL. If you have rust and filth you need to flush it out with radiator cleaner first, if you find OIL that's a whole other story. Check your recovery tank for same contaminants and clean it also.