My suggestion is to wait until you get to Tier 5 before turning in those BNPs for Dilithium. At that point, you can see how many BNPs you need to spend, and plan accordingly.
Don't wanna get up to Tier 5 and find that you need more BNPs after having traded a bunch in. After all, unless you get lucky and find a bonus +10 BNPs in your loot bag, you only get 1 BNP per STF -- it'll take a little while to make those back if you find yourself needing them.
If you want to get all the Space Sets and Ground Sets, then its 105 for the Space Sets (3x30+15) and 90 for the Ground Sets so 195 in total. Thats for the Borg space set and all the space and ground sets for the Omega, MACO/Honour Guard, and Adapted. If you just want to have a space set and a ground set at Mk XII, then its between 45 to 60 depending on which space set you choose.