Remember that everyone has an opinion and you may be more confused than when you started....
Now onto more info:
My experience with silicone o-rings is that they can handle very high and low temperatures. Their downside is that they are not very abrasion resistant. Polyurethane seals tend to be more resistant than other materials. I don't know how they handle steam though.
Quite honestly, unless you're going to run this engine all the time, just go with viton or buna. I use buna in my engines with no problems on either compressed air or live steam. But then they only run for a few minutes at a time.
No material is ideal they are all a compromise of availability, performance and cost. Evaluate how you're going to be using your engine and choose accordingly.
...Ved.
