Congratulations on completing that mammoth venture, Susan.
As for the spin offs that's up to you. However say you start with Torchwood or The Sarah Jane Adventures then you'll have already done the main leads as they've all been in Doctor Who so you'd only be repeating yourself unless you just did those who had never been in Doctor Who which narrows it down a fair bit and many of the actors would be unheard ofFs both then and now appearing in what were spin offs on lesser channels in CBBC and BBC3.