To answer a few of your questions more specifically:
What is the maximum validity?
All Let’s Encrypt certificates are valid for exactly 90 days.
can we enable auto-renewal of the certificates for minimum 2 years?
Auto-renewal is performed by the client software that you use to request your certificate. Whether it works and whether it’s enabled depends on how your software environment is configured. We recommend that everyone always use autorenewal for Let’s Encrypt certificates.
Would you provide clear instructions to install the certificate
This depends on the environment where the certificate is used and is different for every environment, and also for every client application. Hopefully your environment is common enough that there is already documentation somewhere describing a good way of using Let’s Encrypt certificates in that environment.
and if in case of any issues, is there any call support?
It might be that a third-party could provide paid call support; the only support officially offered by Let’s Encrypt itself is this forum.
The resources that Jürgen linked to above are helpful to read to understand more about what Let’s Encrypt offers.