New ElasticSearch setup

We have a 3 node elasticSearch cluster setup on docker instance OS: Oracle Linux. This Elasticsearch version 7.17.6 is used by client application Oracle webcenter which has 7 Million documents stored on the Database.

Recently, due to critical issues, elasticsearch servers had malformed & we are planning to install similar setup of Elasticsearch on new servers so as can be used by our application Webcenter. My queries:
1) What loss can we expect if we move to new servers? (we had been using previous Elasticsearch servers since last 4 years). Will it be only Indexes lost and would it reinstate to our previous elasticsearch servers if we do a full collection rebuild on new servers after complete setup?

2) Can we copy & use any data from our previous ES servers to new e.g. indexes etc since we stil have data intact in them?

3) Should we install our previous Elasticsearch version 7.17.6 or the latest ES version? What difference it will make?