The Storm of Sigmar set is really not the way to go if you are concerned with points and balance.
The way points work in AoS is that you purchase blocks of 1, 3, 5, or 10 models within a unit (unit dependant), one command set and any options for the unit must be noted and are free. This makes army building pretty easy and elegant, but with some armies it requires some angling to get to use all of your points available.
If for instance you had 3 Stormcast Liberators and want to take them as a unit, you may do so, but you still pay for a block of five.
With that in mind, in this box you get all units under strength. Stormcast - 220pts: 3 Liberators (unit at 60% strength) 2 Retributors (unit at 40% strength) Bloodbound - 160pts: 3 Blood Warriors (unit at 60% strength) 5 Bloodreavers (unit at 50% strength)
I would suggest getting the General's Handbook first, it is an excelent resource, asking your nephew which army he likes best, and then getting him the Start Collecting set for the army of his choice. These are great value and very well composed.