Question on Italian meringue icing

I have an order they want Italian meringue icing/frosting (not the buttercream one) First time making this I have a few questions..

-I searched so many sites some call for castor sugar which I found out is finer than regular sugar…is regular sugar just as good to use?
-I would like to make it ahead of time incase it doesn’t turn out, if it works out can I keep it in the fridge for a day few days before I use it? or will it separate.
- Do I really need a candy thermometer?, I do not have one…yet
-If anyone has a easy recipe please send…I will be doing a 10 inch cake so I need it to fill between the 2 layers and cover the whole cake

Thanks for your help!!
Angela