Recommendations For Wall Decals -what questions should I ask a contract sign maker ?

Hi,
I'm calling some local businesses to fullfill my orders and just want to prepare myself to ask the right questions. What I'm doing is setting up a website that will allow customers to pick out artwork that is vectorized and then the size that want. These will be for interior walls of homes, restaurants and businesses. My questions is what is the best material to use for interior wall decals? Any other questions I should ask while shopping for a printer besides materail and sizes they offer? Thanks for any help.