How to design and implement a custom business proces and workflow with multi level request and approval

We are building a custom business process and workflow in our product, with multi level request and approvals which will be highly customized based on the business needs of multiple clients. We are looking for a simple Java based workflow engine which is cost effective and easy to implement.

Also suggest if any of open source or proprietary engines can be integrated into our product seamlessly with minimum effort.

The business process that is initiated must be flexible, and must be able to meet changing business rules of the organization.

The business process must be able to decide between granting access immediately versus introducing manual intervention steps and seeking approval prior to granting access.

The business process must be able to perform validations, including Segregation of Duties (SoD) checks on what is being requested, by who, for whom, and in what context.

If manual intervention is required, then the business process must have the ability to assign to users or groups of users and escalate, reassign, or expire if no response is received in a timely manner.

For manual intervention, the business process must have the ability to gather information from the approvers, including comments and attachments.

The business process must be able to interact with external systems, such as ticketing systems, when automated access grants are not possible, or the organization's rules require that access is granted manually.

All decisions and actions must be audited and available in a reportable manner to allow the organization to measure performance of the process and also for auditors to fulfill compliance requirements

I have heard about BPEL engines but haven't worked on it, will it solve our purpose? Please advice?

https://en.wikipedia.org/wiki/List_of_BPMN_2.0_engines

Thank you in advance