It's tempting to get the starter, the I Am the Master set, and the Second Doctor promo set, but I would not be surprised if more unofficial sculpts come soon. I got a few of the Doctors in a Mystery Box from Warlord ages ago, so that will do me for now.
Between Big Mr. Tong and Crooked Dice, a fair number of options are already covered. Crooked Dice has some solid not-UNIT models including the Brigadier.