A 24059 continuous duty should be fine for your siren or any other accessories. You really shouldn't need an additional solenoid for your starter. However if you want an additional solenoid to save your key switch, a 24023 would be in order. But you will need to have two separate solenoids. My fleet of Diesel trucks are wired like this. A type 24059 for the heater etc and a type 24023 in the starter circuit. From your key switch a wire from the accessory position goes to the 24059 and a wire from the start position goes to the 24023. You cannot use the same solenoid for both accessories and starting. If you do not understand the English, send me a personal message with a phone number and I will explain in German. Translation programs are not very good with technical terms.