there is no separate function to buy & store blocks, all you can do is upgrade bastion and when you click it (if blocks are enough it will start building) else it will show a pop up saying you need this much rubies (to purchase this much blocks) and once you confirm, it will be upgraded.

Thank you.  

Is there a way to know how many rubies/blocks would be needed for each level up to 5?

Thanks

even if you don't have the required  blocks when you click on upgrade it'll give you the option to purchase the remaining blocks. But word to the wise its very expensive. I suggest competing in as many BH tournaments as possible to get them for free. Other than that theres no way to prepurchase blocks.