I have had some experience with polyurethanes - we learned a lot - plenty can go wrong. I do applaud your pragmatic approach and will watch for your progress.
The term 'polyurethane' is about as generic as 'fibreglass'. Rudimentary differences in the chemistry has significant impact on properties. Suppliers have a broad catalog of products and properties to choose from. Era http://www.erapol.com.au/products/cast-elastomers/
in my country is one such company, usually they will supply samples for serious potential customers.
The usual supply format is as a multipart (most commonly 2) resin. Mixing - simple pot mixing with stirrers will require de-gassing in a vacuum pot so requires a longer pot life, the next easiest is to use viscostatic (tubes with a series of mix blades inside) dispenser.
Pre-cast PU's such as sheet can be a mixed bag - pure PU will have a liquid clear appearance in an amber colour - be suspicious of a milky appearance or coloured stock.
Some boutique suppliers of PU formulations exist although I no longer have contact details for them. Tell them your requirements and application and they can provide formulations to suit. Tell them something can't be done in PU is like a red rag to a bull.