It your just looking to build a prototype to check dimensions, work out design faults etc etc then personally I would just use MDF, it's the cheapest of the 3 options & although it does absorb moisture if not sealed properly this happens over time so shouldn't be a problem for you. Same with ply, yes it can warp but generally over time.
What are you using to make the parts?? If your machining them I doubt there is much difference between the 3 but if you are using basic hand tools then again MDF would score over ply as it's easier to cut & drill accurately.