you might struggle with some flexible filaments on the ultimaker.

But both formfutura flexifill and fenner drives semi-flex are a bit stiffer so should work better in a 3mm bowden extruder.

Best material for limbs, casing etc would be pet.
Best for crash skids would be semi-flex.

Printed parts are every bit as strong as moulded parts. As long as they are designed and printed correctly :-)

Pet Tougher, easier to print with and a lot more bend/crash resistant than abs.