1. System Requirements: what are hardware and software requirements for Nxlog Enterprise edition.

The short answer is pretty minimal, though this will depend on your setup. Please see the following link for additional information.
https://nxlog.co/documentation/nxlog-user-guide/requirements.html

2. Available Integrations: what are the inputs supported for the tool(file, database, API, LDAP, etc...)
3. Kafka Output Available: can we forward output to Kafka

Grouping these together. Yes, we support those inputs and Kafka out as well. See the following link for a full list. Much easier than listing them all again here :)
https://nxlog.co/documentation/nxlog-user-guide/available-modules.html

4. Agent Mechanism: With one agent how many logs we can transfer. is it one per system?

There is no physical limit within the software on how many logs you can transfer. One agent can gather many logs and send to many outputs.

5. Customizable: can we customize the code according to the requirements

NXLog EE is not open source. NXLog CE is open source

6. Price: Pricing details

I have forwarded this post to one of our sales guys who will get with you later today to discus pricing.