I'd argue though that the best option is to offer them a dedicated instance of your app and secure access via VPN (as someone else already suggested). Having the app installed within their datacenter will likely require you to have two different deployment models, one for your standard usage and one for their datacenter. It may even require you to make custom code modifications to work around limitations or security settings that do on their "standard" OS images. So once you go into their datacenter, you need to provide prepackaged deployments that have a configuration tool etc. Not saying any of this is insurmountable but you have to make sure you properly charge and expect it to take far longer than you anticipate as has been said.
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This is good advice if you are going to do it.
I'd argue though that the best option is to offer them a dedicated instance of your app and secure access via VPN (as someone else already suggested). Having the app installed within their datacenter will likely require you to have two different deployment models, one for your standard usage and one for their datacenter. It may even require you to make custom code modifications to work around limitations or security settings that do on their "standard" OS images. So once you go into their datacenter, you need to provide prepackaged deployments that have a configuration tool etc. Not saying any of this is insurmountable but you have to make sure you properly charge and expect it to take far longer than you anticipate as has been said.