The one object vs multiple object is a common question and the answer is: It’s a matter of preference. As far as animating it, separate objects if you do objected based rigging/animating(I don’t recommend this). If you use an armature(Recommended) then it will work either way. Now for your second question. Modeling is all about purpose and expected use. I’d say the your method is good and it works, unless you plan on doing closeup or just the caster wheel(s) for your renders.
