Don't go too soft on the plastic you pick.
You'll have flat spot in the wheels if you leave any weight on them overnight.
The flat spots usually go away over time, but its a pain to move a machine while the flat spots are there.
And remember, the wheels are only supporting 1/4 of the total weight.
I'd pick wheels capable of carrying 1/2 the weight or more.
www.harborfreight.com has a decent selection of wheels in varying diameters, materials, and features.
Maybe stop in the store and get your hands on them to see what suits you best.
Lastly, don't forget to get measurements of the mounting flange holes unless you're OK with redrilling and/or welding plates.