Will you need to be mounting hardware on the helicopter itself? Or will the additional hardware and software be at the 'base?'
A micro heli like the Blade MCX2
is light, cheap, easy to fly and inexpensive to repair and replace ... but it doesn't have the oomph to loft custom circuit boards.
Something like the Blade mCP X
has some power to spare, but it harder to fly, less stable, pricier to fix and still not very big.
With a 250 size
, you can start flying around a reasonable payload, but the machine itself is much harder to fly.
What kind of extra payload, if any, will your copter have to carry around?
Have you considered a quadcopter?