All C rank droids are absolutely disgusting aka terrible... they have nearly no sources for their rank up material.
Although despite that - unless you can get your hands on B rank droids - you still need to have at least 14 drones for stat bonuses.... But if you want to buy C rank droids thinking they will be strong - give up that hope, they are worse than D rank ones...
Casual f2p will have all D drones to C rank in around 3 weeks.... and then C>B them in around 1-1.5 more month...
Hardcore f2p will probably only take 1.5 month total to get D rank to B rank. B rank droids are mostly good because they have an extra skill which is stronger than the other skill by far. But let D rank droids reach B rank and they are only slightly worse than initial B rank droids.
Basically this game wants you to progress as fast and as far as possible, so you need to exhaust your premium resources and aim at even higher goal constantly to develop faster instead of conserving your premium for something.
First you want to get 1 costume ASAP to get going with costume lvl ups. Then you need to invest into 3k ether foxy box pulls to ensure you get a lot of A rank gear without wasting much fodder and start focusing on leveling up only your A rank gear. Initial aim is to reach that 1 guaranteed foxy box after 10 3k ether pulls.