Gelt is awesome, Metal's not ideal for an all cav list but it's not bad at all. With Gelt it becomes great. He is basically one piece though and would need conveting to another lore. Metal also gives flaming attacks, good against Hydras, Hell Pits, Terrorgeists
I run a list with a lot of white wolves and lore of beasts works well for them, it makes full use of the lore attribute. I use the model from the Griffon as my figure.
If you're not after a fun game, lore of life is good, especially to heal up the steam tank(s) and resurrect expensive knights. Dwellers also slaughters O&G, Skaven, VC (low strength big units) Chaos (no 3+ ward saves for you!) and DEs, low strength.
As for the others, Light and Shadow are always good. Fire could give you the fire power you lack, good for dealing with skirmishers, monsters, etc.