Get Autodesk Fusion 360, its free for non-commercial use, all cloud based and will cover everything you need to do.

I start by modeling a few of the main components (wheels, motors, rams, cylinders etc) then do a rough bulkhead/baseplate/chassis (depending on your style of build) then just start adding the main bits in. You'll find you wil then do the other parts as and when you need them.