ok the stunts first off are just a random roll... its optional and should be left to the gm's discrestion. To select a caster is 3 shifts... max comp would be 12 shifts. keep in mind that it is a randomized table. so the 12 shifts give you a +12 on your comp roll... meaning if you get a -4 you still get 8 for max. so to summon a max comp caster ghost would be 15 shifts. And he would be a ghost of a caster... so the skill sets are the costs.
As to magic it would be up to the gm or read the below spoiler,
Memories would be what he casts with so the more he casts the weaker he gets
Basically the table allows you to roll for summoned random ghosts. you could use a 1 shift summon to bring up a random type of ghost from comp lvs -4 to +4. the extra shifts are to choose the type and give a bonus on the second roll for comp lv... the idea is your pulling a ghost to you and the end result is at least partially randomized... unless you can sense every ghost in your range then you wouldn't be able to tell exactly what your pulling.
