A couple of thoughts to get you started.
Your 3/4G carrier selection is critical. How well do they cover the coach routes?
Also, you're likely to get the 3/4G out of the coach using a RAP I suspect. Have a careful think about where the unit will be physical installed. What about antenna fix feasibility? Both WiFi and 3/4G.
Be careful when selecting the mobile USB adapter. Aruba have strings for some, others need a lot of work to setup correctly (TAC can help sometimes).
Also, I'm not so sure customers on a coach will be willing to pay. I wouldn't. I would recommend the customer consider this a cost of sale and value add / business differentiator over their competition. Unless they know their competition already charges for this on their coaches? I would think it's difficult to justify the charges, based on the fact the 3G might not always give consistent service and quality of delivery.
Just a thought.