Why you want to use the very old version 10.0 instead of the recent 10.5 (or soon released 10.7) for your POC?
The tools you need depend on the “Java” client you want to build and the how you build it.
e,g, when you use the integration server via service calls using JSON, SOAP, REST or plain HTTPS calls with
an XML payload the best matching test tool depends on this. Likely SOAP UI or POSTMAN.
Are you sure than an ONPREM installation is what you need or would be an cloud solution (e.g. WM.IO) be the better solution?
There are free trials available. Also when you want to use an API, maybe not only one in the future, maybe you need an API management solution (e.g. API Gateway, also as free trial available).
When you are use you need ONPREM, please see YOUTUBE for examples:
Alternatively apply for an free trial here: https://www.softwareag.cloud/site/product/webmethods-io-integration.html#/