I don't think you'll be able to give him both Cupped Hands and Plaque of Tepok, as they're both Arcane items and it can take only one (unless that has changed in 8th).
Here's how my Slann, Lord Magwe, will run in 8th edition if he is by himself:
Focus of Mystery- no chance of me not getting the good spell I really want
Focused Rumination- the best discipline we have access to in my opinion, never leave home without it
Becalming Cogitation- Yes, it will prevent the enemy from miscasting. But, it could also keep him from getting a spell off in the first place, which means he's done casting for the turn. Some of those mega spells like Purple Sun and the Skaven super spell are worth miscasting to get off, and I'd rather not be on the receiving end.
Divine Plaque of Protection- 2+ ward save against nearly any ranged attack for only 30 points. This covers magic and non-magic attacks, so just stay out of close combat and you're golden. For me, this is a nice alternative to Higher State (ethereal) and Unfathomable (MR 3) at 50 points apiece, neither of which will protect you against something like Warplock Jezzails, Dwarven artillery with magic runes, or Tzeentch flamers.
Cupped Hands- throw all your dice at a super spell, lose control, then bounce the effects of the miscast onto his wizard on a 2+. Can't beat that with a stick.
There has been a lot of discussion about Temple Guard, and I agree they're still very usable if the Slann takes lore of Life. One new spell helps you ignore miscasts, and another restores slain models to your unit, so, barring a complete disaster, the guards will be able to defend you to the bitter end with their S5 halberds.
In the case of using the Temple Guard, I think Unfathomable Presence is a must have, as it gives great protection to the whole unit.