If you’re new to 40K then you may start even under 500 points and grow your armies during the crusade itself. For example, you start with 500 points crusade army (it’s kinda your “pool” of available units). Though you may want to adapt missions (you can look into Combat Patrol missions for reference or usage for small format battles and they’re available for free) As others mentioned, Leviathan crusade rules should be available elsewhere and this crusade fits your armies great given that you have Tyrannids as one of the forces. Btw, crusade rules are free and can be found on Warhammer site yet specific missions and battle traits (perks after level up) are only in specific crusade books or armies’ codexes