Network configuration question

I would appreciate any help on the following scenario as I have not worked with this type of network configuration before.

The client has multiple businesses throughout three buildings. Building A houses a retail store. It has it's own internet connection and router (internal network of 192.168.0.x). Buildings B and C house the other businesses and are connected via fiber between the buildings (internal network of 192.168.1.x). Buildings B and C also have their own internet connection and router. Building A is connected to building B via a point to point wireless solution.

The desired result is that a few of the users in building B need to access resources in building A along with all of the resources in buildings B and C. One user in building C needs to access resources in Building A.

The accounting department is concerned that the users in building A will have access to the server that is for the companies in building B and C.

So I need help setting up a proper configuration that will work in this scenario. It would be ok if all three buildings were in the same network range (192.168.1.x for example) as security needs to be handled via other means than just different IP addresses. I need users in building A to access the internet via the router in building A and users in buildings B and C to access the internet with the router in building C. It would be great if there could be failover so that if one of the internet connections failed, the traffic would route through the network to the other router.

I appreciate any guidance that you may have to offer.

 

Managed switches, creative routing & VLans....
To get anymore in depth I'd need a consulting fee.