What lore would be best do you think? Wandering deliberations? Focus of mystery? Light or life for buffs?
That really comes down to many things. Your personal play style, army composition and in some cases the opposing army you are facing. Are you looking to pick a lore for an all-comers list or tailor it against a specific opponent/army? For instance Lore of Metal is amazing against WoC, but not nearly as good against Elves. Alternatively you can choose a lore that focuses on buffing your own forces regardless of the enemy (Light or Life).
Typically, I believe the best single lores for us (in no particular order) are Life, Light, Shadow and Death. In the case of your flying solo Slann, be aware that the Life signature spell will be useless (with the exception of the effect of the lore attribute), although being able to heal yourself up is nice. A flying Slann is uniquely capable of making the most of the lore of Death because it can get into optimal range...
however be very wary of Purple Sun misfiring and taking out your Slann (because he has no Look Out Sir roll to save him). Light is always a solid choice for Lizardmen because it helps offset our weaknesses of poor initiative and weapon skill. Shadow is also a great lore with many great hexes and a solid #6 spell.
A lot of people really like Wandering Deliberations. It gives you a very versatile tool chest of spells, with good casting values. You really can't go wrong with this option, but you will be lacking that uber #6 killer spell.
And last but not least, my personal favourite, Focus of Mystery. It does take a bit of practice to master, but it is an amazing choice. The lore of high magic already offers you a great deal of utility on its own, but on top of that you get to exchange spells mid-game. That is a potential game changer in my opinion. Arcane Unforging is great against armies with killer magic items... in your case specifically the WoC Lord's 3+ ward save or the High Elves' Banner of the World Dragon. That alone is massive. Walk Between Worlds is also amazing and Fiery Convocation can disintegrate squishy expensive Elf units (as well as giant horde units that other armies field). You've also go a great magic missile and a spell to heal wounds. Tempest is probably the weakest of the bunch, I would cast it early and trade it. Remember that when you take High Magic, you ALWAYS take it with Focus of Mystery. I find the following tactica video to be very helpful:
