Hi there.

Magento has a bunch of great payment gateway choices on their website/in their app store.

Each can easily be integrated to Magento 2.0 + Sage 500 by a good integration vendor, like eBridge Connections.

Just Google them. Good luck!

We also use eBridge to link Magento and Sage 500. They do alright work, the only real problem is that they seem to have difficulty feeding information from Sage 500 (such as tracking numbers) back into Magento after shipment.

I could look up some bills if you want to know costs.