I play some Necrons when not burninating the Enemies of Man with my Sisters of Battle, so I'll wish you good luck getting rid of the current infestation of the living
But yeah, you may have some issues getting through tougher things with that setup.
For what it's worth, I recommend grabbing a Start Collecting box next - and soon - as for a good price it gets you another 12 Warriors, the Overlord, and a bit of variety. 5 Immortals (Tesla is prolly better but I LIKE my Gauss dammit... though I suppose you COULD build them as Deathmarks) and either a Catacomb Command Barge or Annihilation Barge depending on what you build it as.
I added the 'and soon' as the prices of Start Collecting will go up February 4th
Ask not the Eldar a question, for they will give you three answers; all of which are true and horrifying to know.