I am operating on a smaller basis, building and painting fully before purchasing a new batch. My first batch is finished and painted and im building up my hobby bankroll before I buy my next batch. As it is I have a lot of room for allies and take max 400 points every round to just field my army at 2k. So far Allies that I have taken are Daemonettes, Chaos Sorcerer lord, keeper of secrets, The Masque of Slaanesh, and Ogroid Thamategure(Not spelled correctly lol)
The all-stars were definitely the Daemonettes and the Sorcerer. I'm currently light on bodies, only running 20 fireglaives and 9 Bull Centaurs. The Daemonettes in either blocks of 30 or 20 with hero, have been instrumental in winning some of my matches. They hit HARD on the charge, but fold under pressure quite well as they do not have an invuln like their 40K counterparts. Being able to run a big block of Bulls on one flank and Daemonettes on the other force the opponent to split his deployment.
The Chaos Sorcerer has been amazing with my big block of Bulls, Rerolling 1's for hit, wound and saves are everything this unit needs to excel.
I have read reports of Gutrot Spume and a unit of Blightlords as your allies, but I have not tested them yet.