I'd cast first, it's not even uncommon to cast before anything else, it may even take a year to cast if you can't find the right actor, so I'd get that together and if you can complete your cast and your production is ready then you secure your locations, but these need to be scouted first of course, and you can make tentative arrangements, so you can do this anytime, even while casting. Just ask their availability same as you would to the actors.